Electrolyte, electrochemical device, secondary cell, and module

ABSTRACT

Provided is an electrolyte solution for secondary batteries that are less likely to generate gas and excellent in high-temperature storage characteristics; an electrochemical device using the electrolyte solution; and a module using the electrochemical device. The electrolyte solution contains a solvent and an electrolyte salt, the solvent containing a fluorine-containing acyclic carbonate represented by the formula (1) and a fluorine-containing succinic anhydride represented by the formula (2). The electrolyte solution contains not less than 0.001 mass % but less than 90 vol % of the fluorine-containing acyclic carbonate, and the electrolyte solution contains 0.001 to 20 mass % of the fluorine-containing succinic anhydride.

TECHNICAL FIELD

The present invention relates to electrolyte solutions, electrochemical devices, secondary batteries, and modules.

BACKGROUND ART

Current electric appliances demonstrate a tendency to have a reduced weight and a smaller size, which leads to development of lithium ion secondary batteries having a high energy density. Further, lithium ion secondary batteries are used in more various fields, and thus are desired to have improved battery characteristics.

For practical use of lithium ion secondary batteries, improvement in the storage characteristics and cycle characteristics will be more and more important. Many efforts have been made to improve the battery characteristics of lithium ion secondary batteries especially by focusing on electrolyte solutions. Some studies are given as examples in the following.

Patent Literature 1, aiming to provide a non-aqueous electrolyte secondary battery with high charge and discharge efficiency and excellent cycle characteristics, discloses an electrolyte solution that contains a lithium salt and 0.5 to 10 wt % of a cyclic acid anhydride dissolved in a mixed non-aqueous solvent that contains a cyclic carbonate and an acyclic carbonate (70 vol % or more as a total amount in the whole amount of the solvent). Use of this electrolyte solution minimizes decomposition of the electrolyte solution contained in a non-aqueous electrolyte secondary battery that includes a graphite-based negative electrode.

Patent Literature 2, aiming to provide a non-aqueous electrolyte battery with excellent safety, high charge and discharge efficiency, and high energy density, discloses a non-aqueous electrolyte that contains at least propylene carbonate and contains 0.01 to 20 mass % of a fluoride of a cyclic carboxylic anhydride relative to the whole mass of the non-aqueous electrolyte. Use of this electrolyte suppresses decomposition of an organic solvent constituting a non-aqueous electrolyte solution and hinders oxidative decomposition of the fluoride of a cyclic carboxylic anhydride.

Patent Literature 3, aiming to provide a battery that gives small change over time from the initial battery characteristics after long-term storage, discloses a non-aqueous electrolyte that contains a specific organic carboxylic anhydride. Use of this electrolyte controls the generation form of a protective film on the surface of a negative electrode.

Patent Literature 4, aiming to provide a non-aqueous electrolyte secondary battery that has high charge capacity, excellent characteristics over a long term, and especially excellent discharge capacity retention ratio, discloses use of a non-aqueous electrolyte solution in a non-aqueous electrolyte secondary battery containing a negative electrode active material that includes at least one atom selected from the group consisting of a Si atom, a Sn atom, and a Pb atom. The non-aqueous electrolyte solution contains a carbonate containing at least one of an unsaturated bond and a halogen atom and an acyclic/cyclic carbonate having a specific structure.

CITATION LIST Patent Literature

Patent Literature 1: JP 3658517 B

Patent Literature 2: JP 4415521 B

Patent Literature 3: JP 2006-66320 A

Patent Literature 4: JP 2007-299541 A

SUMMARY OF INVENTION Technical Problem

Still, there is room for studying what kind of component(s) in what amount an electrolyte solution should contain to further improve the battery characteristics.

Further improvement is required for solving some problems. For example, diols or monoalcohols contained in a high permittivity solvent (e.g., ethylene carbonate) or a low viscosity solvent (e.g., dimethyl carbonate) gradually react with a fluorine-containing electrolyte at room temperature to generate hydrogen fluoride (HF) gas, and the amount of the gas increases with time to deteriorate the cycle characteristics of the battery.

The present invention was made in view of the current situation in the art and aims to provide an electrolyte solution for producing an electrochemical device such as a secondary battery that is less likely to generate gas and has excellent high-temperature storage characteristics; an electrochemical device such as a secondary battery using the electrolyte solution; and a module using the electrochemical device.

Solution to Problem

Through intensive studies for solving the above problem, the inventors found that use of an electrolyte solution that contains, as a solvent, a specific fluorine-containing acyclic carbonate and a specific fluorine-containing succinic anhydride at a specific composition unexpectedly suppresses generation of gas to achieve an electrochemical device such as a secondary battery with excellent high-temperature storage characteristics, and thereby completed the present invention.

In other words, an aspect of the present invention is an electrolyte solution containing a solvent and an electrolyte salt, the solvent containing a fluorine-containing acyclic carbonate represented by the formula (1) and a fluorine-containing succinic anhydride represented by the formula (2), the electrolyte solution containing not less than 0.001 mass % but less than 90 vol % of the fluorine-containing acyclic carbonate, the electrolyte solution containing 0.001 to 20 mass % of the fluorine-containing succinic anhydride, the formula (1) being:

Rf¹OCOORf²  (1)

wherein Rf¹ and Rf² are the same as or different from each other and individually represent a C1-C4 alkyl group or a C1-C4 fluorine-containing alkyl group, but at least one of Rf¹ and Rf² is C1-C4 fluorine-containing alkyl group, the formula (2) being:

wherein Rf³ to Rf⁶ are the same as or different from one another and individually represent a hydrogen atom, a fluorine atom, or a C1-C10 fluorine-containing alkyl group, but not all of Rf³ to Rf⁶ are hydrogen atoms.

The fluorine-containing acyclic carbonate represented by the formula (1) is preferably at least one selected from the group consisting of CF₃CH₂OCOOCH₂CF₃, CF₃CH₂OCOOCH₃, and CF₃CF₂CH₂OOCOOCH₂CF₂CF₃.

The fluorine-containing succinic anhydride is preferably at least one selected from the group consisting of compounds represented by the following formulas (3) to (5):

The solvent preferably further contains at least one selected from the group consisting of a non-fluorinated saturated cyclic carbonate, a fluorinated saturated cyclic carbonate, and a non-fluorinated acyclic carbonate.

The electrolyte salt is preferably at least one lithium salt selected from the group consisting of LiPF₆, LiBF₄, LiSO₃CF₃, LiN(SO₂CF₃)₂, LiN(SO₂C₂F₅)₂, lithium difluoro(oxalato)borate, lithium bis(oxalato)borate, and a salt represented by LiPF_(a)(C_(n)F_(2n+1))_(6-a) wherein a represents an integer of 0 to 5 and n represents an integer of 1 to 6.

Another aspect of the present invention is an electrochemical device containing the electrolyte solution.

Still another aspect of the present invention is a secondary battery containing the electrolyte solution.

Still another aspect of the present invention is a module containing the electrochemical device or the secondary battery.

Advantageous Effects of Invention

Use of the electrolyte solution of the present invention enables production of an electrochemical device such as a secondary battery that is less likely to generate gas and has excellent high-temperature storage characteristics.

DESCRIPTION OF EMBODIMENTS

The electrolyte solution of the present invention contains a solvent and an electrolyte salt. The solvent contains a fluorine-containing acyclic carbonate represented by the formula (1) and a fluorine-containing succinic anhydride represented by the formula (2), the formula (1) being:

Rf¹OCOORf²  (1)

wherein Rf¹ and Rf² are the same as or different from each other and individually represent a C1-C4 alkyl group or a C1-C4 fluorine-containing alkyl group, but at least one of Rf¹ and Rf² is C1-C4 fluorine-containing alkyl group; the formula (2) being:

wherein Rf³ to Rf⁶ are the same as or different from one another and individually represent a hydrogen atom, a fluorine atom, or a C1-C10 fluorine-containing alkyl group, but not all of Rf³ to Rf⁶ are hydrogen atoms.

In the fluorine-containing acyclic carbonate represented by the formula (1), Rf¹ and Rf² are the same as or different from each other and individually represent a C1-C4 alkyl group or a C1-C4 fluorine-containing alkyl group, but at least one of Rf¹ and Rf² is a C1-C4 fluorine-containing alkyl group.

The above carbon number is preferably 1 to 3 in order to achieve good compatibility with the electrolyte solution.

Examples of Rf¹ include CF₃—, CF₃CF₂—, (CF₃)₂CH—, CF₃CH₂—, C₂F₅CH₂—, HCF₂CH₂—, HCF₂CF₂CH₂—, and CF₃CFHCF₂CH₂—. Preferred among these are CF₃CH₂— and HCF₂CH₂— in terms of high flame retardance, good rate characteristics, and good oxidation resistance.

Examples of Rf² include CF₃—, CF₃CF₂—, (CF₃)₂CH—, CF₃CH₂—, C₂F₅CH₂—, HCF₂CH₂—, HCF₂CF₂CH₂—, and CF₃CFHCF₂CH₂—. Preferred among these are CF₃CH₂— and HCF₂CH₂— in terms of high flame retardance, good rate characteristics, and good oxidation resistance.

Specific examples of the fluorine-containing acyclic carbonate represented by the formula (1) include fluorine-containing acyclic carbonates such as CF₃CH₂OCOOCH₂CF₃, CF₃CH₂OCOOCH₃, CF₃CF₂CH₂OCOOCH₂CF₂CF₃, and CF₃CF₂CH₂OCOOCH₃. Examples thereof further include compounds disclosed in JP H06-21992 A, JP 2000-327634 A, and JP 2001-256983 A. Preferred among these is at least one compound selected from the group consisting of CF₃CH₂OCOOCH₂CF₃, CF₃CH₂OCOOCH₃, and CF₃CF₂CH₂OCOOCH₂CF₂CF₃ in terms of high effectiveness of suppressing generation of gas to improve the high-temperature storage characteristics. The fluorine content of the fluorine-containing acyclic carbonate is preferably 20 mass % or more, more preferably 30 mass % or more. The fluorine content is preferably 60 mass % or less, more preferably 55 mass % or less.

The fluorine content may be calculated based on the structural formula of the fluorine-containing acyclic carbonate by

{(number of fluorine atoms×19)/(molecular weight of fluorine-containing acyclic carbonate)}×100(%).

The electrolyte solution of the present invention contains not less than 0.001 mass % but less than 90 vol % of the fluorine-containing acyclic carbonate. Too large or too small an amount of the fluorine-containing acyclic carbonate fails to suppress generation of gas and to improve the high-temperature storage characteristics. The amount of the fluorine-containing acyclic carbonate in the electrolyte solution is preferably 0.01 mass % or more, more preferably 0.1 mass % or more, while preferably 75 vol % or less, more preferably 70 vol % or less.

In the fluorine-containing succinic anhydride represented by the formula (2), Rf³ to Rf⁶ are the same as or different from one another and individually represent a hydrogen atom, a fluorine atom, or a C1-C10 fluorine-containing alkyl group, but not all of Rf³ to Rf⁶ are hydrogen atoms.

The above carbon number is preferably 1 to 3 in order to achieve good compatibility with the electrolyte.

Specific examples of Rf³ to Rf⁶ include H—, F—, CF₃—, CF₃CF₂—, CH₂FCH₂—, and CF₃CF₂CF₂—. Preferably, at least one of Rf³ to Rf⁶ is F— or CF₃— in order to achieve high oxidation resistance and good compatibility with the electrolyte solution. More preferably, Rf³ to Rf⁵ are the same as or different from one another and individually represent H— or F—, and Rf⁶ is F— or CF₃—.

Specific examples of the fluorine-containing succinic anhydride represented by the formula (2) include monofluorosuccinic anhydride, tetrafluorosuccinic anhydride, trifluorosuccinic anhydride, 1,2-difluorosuccinic anhydride, and trifluoromethyl succinic anhydride. Preferred among these is at least one compound selected from the group consisting of compounds represented by the formulas (3) to (5).

The electrolyte solution of the present invention contains 0.001 to 20 mass % of the fluorine-containing succinic anhydride. Too large or too small an amount of the fluorine-containing succinic anhydride fails to suppress generation of gas and to improve the high-temperature storage characteristics. The amount of the fluorine-containing succinic anhydride is preferably 0.01 mass % or more, more preferably 0.1 mass % or more, while preferably 8 mass % or less, more preferably 6 mass % or less.

The solvent may further contain at least one selected from the group consisting of a non-fluorinated saturated cyclic carbonate, a fluorinated saturated cyclic carbonate, and a non-fluorinated acyclic carbonate. Alternatively, the solvent may contain at least one selected from the group consisting of a non-fluorinated saturated cyclic carbonate and a fluorinated saturated cyclic carbonate; and a non-fluorinated acyclic carbonate. Alternatively, the solvent may contain a non-fluorinated saturated cyclic carbonate and a non-fluorinated acyclic carbonate.

The solvent is preferably a non-aqueous solvent and the electrolyte solution of the present invention is preferably a non-aqueous electrolyte solution.

Examples of the non-fluorinated saturated cyclic carbonates include ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate.

In order to achieve a high permittivity and a suitable viscosity, the non-fluorinated saturated cyclic carbonate is preferably at least one compound selected from the group consisting of ethylene carbonate, propylene carbonate, and butylene carbonate.

The non-fluorinated saturated cyclic carbonate may comprise one of the above compounds or may comprise two or more thereof in combination.

The fluorinated saturated cyclic carbonate is a saturated cyclic carbonate with a fluorine atom attached thereto. Specific examples thereof include a fluorinated saturated cyclic carbonate (A) represented by the following formula (A):

wherein X¹ to X⁴ may be the same as or different from each other, and individually represent a fluorinated alkyl group which may optionally have —H, —CH₃, —F, or an ether bond, or represent a fluorinated alkoxy group which may optionally have an ether bond; at least one of X¹ to X⁴ is a fluorinated alkyl group which may optionally have —F or an ether bond, or a fluorinated alkoxy group which may optionally have an ether bond.

If the electrolyte solution of the present invention contains a fluorinated saturated cyclic carbonate (A) and is applied to a lithium ion secondary battery, a stable film is formed on the negative electrode so that side reactions of the electrolyte solution on the negative electrode may sufficiently be suppressed. As a result, significantly stable, excellent charge and discharge characteristics can be achieved.

The term “ether bond” herein means a bond represented by —O—.

In order to achieve a good permittivity and oxidation resistance, one or two of X¹ to X⁴ in the formula (A) is/are preferably a fluorinated alkyl group which may optionally have —F or an ether bond or a fluorinated alkoxy group which may optionally have an ether bond.

In anticipation of a decrease in the viscosity at low temperatures, an increase in the flash point, and improvement in the solubility of the electrolyte salt, X¹ to X⁴ in the formula (A) each preferably represent —H, —F, a fluorinated alkyl group (a), a fluorinated alkyl group (b) having an ether bond, or a fluorinated alkoxy group (c).

The fluorinated alkyl group (a) is an alkyl group in which at least one hydrogen atom is replaced by a fluorine atom. The fluorinated alkyl group (a) preferably has a carbon number of 1 to 20, more preferably 2 to 17, still more preferably 2 to 7, particularly preferably 2 to 5.

If the carbon number is too large, the low-temperature characteristics may be poor and the solubility of the electrolyte salt may be low. If the carbon number is too small, the solubility of the electrolyte salt may be low, the discharge efficiency may be low, and the viscosity may be high, for example.

Examples of the fluorinated alkyl group (a) which has a carbon number of 1 include CFH₂—, CF₂H—, and CF₃—.

In order to achieve a good solubility of the electrolyte salt, preferred examples of the fluorinated alkyl group (a) which has a carbon number of 2 or greater include fluorinated alkyl groups represented by the following formula (a-1):

R¹—R²—  (a-1)

wherein R¹ represents an alkyl group which may optionally have a fluorine atom and which has a carbon number of 1 or greater; R² represents a C1-C3 alkylene group which may optionally have a fluorine atom; and at least one of R¹ and R² has a fluorine atom.

R¹ and R² each may further have an atom other than the carbon atom, hydrogen atom, and fluorine atom.

R¹ is an alkyl group which may optionally have a fluorine atom and which has a carbon number of 1 or greater. R¹ preferably represents a C1-C16 linear or branched alkyl group. The carbon number of R¹ is more preferably 1 to 6, still more preferably 1 to 3.

Specifically, for example, CH₃—, CH₃CH₂—, CH₃CH₂CH₂—, CH₃CH₂CH₂CH₂—, and the groups represented by the following formulas:

may be mentioned as a linear or branched alkyl group for R¹.

If R¹ is a linear alkyl group having a fluorine atom, examples thereof include CF₃—, CF₃CH₂—, CF₃CF₂—, CF₃CH₂CH₂—, CF₃CF₂CH₂—, CF₃CF₂CF₂—, CF₃CH₂CF₂—, CF₃CH₂CH₂CH₂—, CF₃CF₂CH₂CH₂—, CF₃CH₂CF₂CH₂—, CF₃CF₂CF₂CH₂—, CF₃CF₂CF₂CF₂—, CF₃CF₂CH₂CF₂—, CF₃CH₂CH₂CH₂CH₂—, CF₃CF₂CH₂CH₂CH₂—, CF₃CH₂CF₂CH₂CH₂—, CF₃CF₂CF₂CH₂CH₂—, CF₃CF₂CF₂CF₂CH₂—, CF₃CF₂CH₂CF₂CH₂—, CF₃CF₂CH₂CH₂CH₂CH₂—, CF₃CF₂CF₂CF₂CH₂CH₂—, CF₃CF₂CH₂CF₂CH₂CH₂—, HCF₂—, HCF₂CH₂—, HCF₂CF₂—, HCF₂CH₂CH₂—, HCF₂CF₂CH₂—, HCF₂CH₂CF₂—, HCF₂CF₂CH₂CH₂—, HCF₂CH₂CF₂CH₂—, HCF₂CF₂CF₂CF₂—, HCF₂CF₂CH₂CH₂CH₂—, HCF₂CH₂CF₂CH₂CH₂—, HCF₂CF₂CF₂CF₂CH₂—, HCF₂CF₂CF₂CF₂CH₂CH₂—, FCH₂—, FCH₂CH₂—, FCH₂CF₂—, FCH₂CF₂CH₂—, FCH₂CF₂CF₂—, CH₃CF₂CH₂—, CH₃CF₂CF₂—, CH₃CH₂CH₂—, CH₃CF₂CH₂CF₂—, CH₃CF₂CF₂CF₂—, CH₃CH₂CF₂CF₂—, CH₃CF₂CH₂CF₂CH₂—, CH₃CF₂CF₂CF₂CH₂—, CH₃CF₂CF₂CH₂CH₂—, CH₃CH₂CF₂CF₂CH₂—, CH₃CF₂CH₂CF₂CH₂CH₂—, HCFClCF₂CH₂—, HCF₂CFClCH₂—, HCF₂CFClCF₂CFClCH₂—, and HCFClCF₂CFClCF₂CH₂—.

If R¹ is a branched alkyl group having a fluorine atom, those represented by the following formulas:

may be preferably mentioned. If the group has a branch represented by —CH₃ or —CF₃, for example, the viscosity is likely to be high. Thus, the number of such branches is more preferably small (one) or zero.

R² represents a C1-C3 alkylene group which may optionally have a fluorine atom. R² may be a linear or branched group. Examples of a minimum structural unit constituting such a linear or branched alkylene group are shown below. R² is constituted by one or combination of these units.

(i) Linear Minimum Structural Units

—CH₂—, —CHF—, —CF₂—, —CHCl—, —CFCl—, —CCl₂—

(ii) Branched Minimum Structural Units

Preferred among these exemplified units are Cl-free structural units because such units are not dehydrochlorinated by a base, and thus are more stable.

If R² is a linear group, the group consists only of any of the above linear minimum structural units, and it is preferably —CH₂—, —CH₂CH₂—, or CF₂—. In order to further improve the solubility of the electrolyte salt, —CH₂— or —CH₂CH₂— is more preferred.

If R² is a branched group, the group comprises at least one of the above branched minimum structural units. Preferred examples thereof include those represented by the formula: —(CX^(a)X^(b))— (wherein X^(a) represents H, F, CH₃, or CF₃; X^(b) represents CH₃ or CF₃; if X^(b) is CF₃, X^(a) is H or CH₃). Such groups can further improve the solubility of the electrolyte salt.

For example, CF₃CF₂—, HCF₂CF₂—, H₂CFCF₂—, CH₃CF₂—, CF₃CF₂CF₂—, HCF₂CF₂CF₂—, H₂CFCF₂CF₂—, CH₃CF₂CF₂—, and those represented by the following formulas:

may be mentioned as a preferred fluorinated alkyl group (a).

The fluorinated alkyl group (b) having an ether bond is an alkyl group which has an ether bond and in which at least one hydrogen atom is replaced by a fluorine atom. The fluorinated alkyl group (b) having an ether bond preferably has a carbon number of 2 to 17. If the carbon number is too large, the fluorinated saturated cyclic carbonate (A) may have a high viscosity, and also the number of fluorine-containing groups increases. Thus, the solubility of the electrolyte salt may be poor due to reduction in the permittivity, and the compatibility with other solvents may be poor. Accordingly, the carbon number of the fluorinated alkyl group (b) having an ether bond is preferably 2 to 10, more preferably 2 to 7.

The alkylene group which constitutes the ether segment of the fluorinated alkyl group (b) having an ether bond may be a linear or branched alkylene group. Examples of a minimum structural unit constituting such a linear or branched alkylene group are shown below.

(i) Linear Minimum Structural Units

—CH₂—, —CHF—, —CF₂—, —CHCl—, —CFCl—, —CCl₂—

(ii) Branched Minimum Structural Units

The alkylene group may be constituted by one of these minimum structural units alone, or may be constituted by a combination of linear units (i), of branched units (ii), or of a linear unit (i) and a branched unit (ii). Preferred examples will be mentioned in detail later.

Preferred among these exemplified units are Cl-free structural units because such units are not dehydrochlorinated by a base, and thus are more stable.

Still more preferred examples of the fluorinated alkyl group (b) having an ether bond include those represented by the following formula (b-1):

R³—(OR⁴)_(n1)—  (b-1)

wherein R³ represents preferably a C1-C6 alkyl group which may optionally have a fluorine atom; R⁴ represents preferably a C1-C4 alkylene group which may optionally have a fluorine atom; n1 is an integer of 1 to 3; and at least one of R³ and R⁴ has a fluorine atom.

Examples of the groups for R³ and R⁴ include the following, and any appropriate combination of these groups can provide the fluorinated alkyl group (b) having an ether bond represented by the formula (b-1). Still, the groups are not limited thereto.

(1) R³ is preferably an alkyl group represented by the following formula: X^(c) ₃C—(R⁵)_(n2)— (wherein three X^(c)'s may be the same as or different from each other, and individually represent H or F; R⁵ represents a C1-C5 alkylene group which may optionally have a fluorine atom; and n2 is 0 or 1).

If n2 is 0, R³ may be CH₃—, CF₃—, HCF₂—, or H₂CF—, for example.

If n2 is 1, specific examples of a linear group for R³ include CF₃CH₂—, CF₃CF₂—, CF₃CH₂CH₂—, CF₃CF₂CH₂—, CF₃CF₂CF₂—, CF₃CH₂CF₂—, CF₃CH₂CH₂CH₂—, CF₃CF₂CH₂CH₂—, CF₃CH₂CF₂CH₂—, CF₃CF₂CF₂CH₂—, CF₃CF₂CF₂CF₂—, CF₃CF₂CH₂CF₂—, CF₃CH₂CH₂CH₂CH₂—, CF₃CF₂CH₂CH₂CH₂—, CF₃CH₂CF₂CH₂CH₂—, CF₃CF₂CF₂CH₂CH₂—, CF₃CF₂CF₂CF₂CH₂—, CF₃CF₂CH₂CF₂CH₂—, CF₃CF₂CH₂CH₂CH₂CH₂—, CF₃CF₂CF₂CF₂CH₂CH₂—, CF₃CF₂CH₂CH₂CH₂CH₂—, HCF₂CH₂—, HCF₂CF₂—, HCF₂CH₂CH₂—, HCF₂CF₂CH₂—, HCF₂CH₂CF₂—, HCF₂CF₂CH₂CH₂—, HCF₂CH₂CF₂CH₂—, HCF₂CF₂CF₂CF₂—, HCF₂CF₂CH₂CH₂CH₂—, HCF₂CH₂CF₂CH₂CH₂—, HCF₂CF₂CF₂CF₂CH₂—, HCF₂CF₂CF₂CF₂CH₂CH₂—, FCH₂CH₂—, FCH₂CF₂—, FCH₂CF₂CH₂—, CH₃CF₂—, CH₃CH₂—, CH₃CF₂CH₂—, CH₃CF₂CF₂—, CH₃CH₂CH₂—, CH₃CF₂CH₂CF₂—, CH₃CF₂CF₂CF₂—, CH₃CH₂CF₂CF₂—, CH₃CH₂CH₂CH₂—, CH₃CF₂CH₂CF₂CH₂—, CH₃CF₂CF₂CF₂CH₂—, CH₃CF₂CF₂CH₂CH₂—, CH₃CH₂CF₂CF₂CH₂—, CH₃CF₂CH₂CF₂CH₂CH₂—, and CH₃CH₂CF₂CF₂CH₂CH₂—.

If n2 is 1, those represented by the following formulas:

may be mentioned as a branched group for R³.

If the group for R³ has a branch such as —CH₃ or —CF₃, the viscosity is likely to be high. Thus, the group for R³ is more preferably a linear group.

(2) In the segment —(OR⁴)_(n1)— of the formula (b-1), n1 is an integer of 1 to 3, preferably 1 or 2. If n1 is 2 or 3, R⁴'s may be the same as or different from each other.

Preferred specific examples of the group for R⁴ include the following linear or branched groups.

Examples of the linear groups include —CH₂—, —CHF—, —CF₂—, —CH₂CH₂—, —CF₂CH₂—, —CF₂CF₂—, —CH₂CF₂—, —CH₂CH₂CH₂—, —CH₂CH₂CF₂—, —CH₂CF₂CH₂—, —CH₂CF₂CF₂—, —CF₂CH₂CH₂—, —CF₂CF₂CH₂—, —CF₂CH₂CF₂—, and —CF₂CF₂CF₂—.

Those represented by the following formulas:

may be mentioned as branched groups.

The fluorinated alkoxy group (c) is an alkoxy group in which at least one hydrogen atom is replaced by a fluorine atom. The fluorinated alkoxy group (c) preferably has a carbon number of 1 to 17. The carbon number is more preferably 1 to 6.

The fluorinated alkoxy group (c) is particularly preferably a fluorinated alkoxy group represented by the formula: X^(d) ₃C—(R⁶)_(n3)—O— (wherein three X^(d)'s may be the same as or different from each other, and individually represent H or F; R⁶ preferably represents a C1-C5 alkylene group which may optionally have a fluorine atom; n3 is 0 or 1; and any of the three X^(d)'s contain a fluorine atom).

Specific examples of the fluorinated alkoxy group (c) include fluorinated alkoxy groups in which an oxygen atom is bonded to an end of the alkyl group for R¹ in the formula (a-1).

The fluorinated alkyl group (a), the fluorinated alkyl group (b) having an ether bond, and the fluorinated alkoxy group (c) in the fluorinated saturated cyclic carbonate (A) each preferably have a fluorine content of 10 mass % or more. If the fluorine content is too low, an effect of increasing the flash point may not be sufficiently achieved. Thus, the fluorine content is more preferably 20 mass % or more, still more preferably 30 mass % or more. The upper limit thereof is usually 85 mass %.

The fluorine content of each of the fluorinated alkyl group (a), the fluorinated alkyl group (b) having an ether bond, and the fluorinated alkoxy group (c) is a value calculated by the following formula:

{(Number of fluorine atoms×19)/(formula weight of the formula)}×100(%)

based on the corresponding structural formula.

In order to achieve a good permittivity and oxidation resistance, the fluorine content in the whole fluorinated saturated cyclic carbonate (A) is preferably 5 mass % or more, more preferably 10 mass % or more. The upper limit thereof is usually 76 mass %.

The fluorine content in the whole fluorinated saturated cyclic carbonate (A) is a value calculated based on the structural formula of the fluorinated saturated cyclic carbonate (A) by the following formula:

{(Number of fluorine atoms×19)/(molecular weight of fluorinated saturated cyclic carbonate (A))}×100(%).

Specific examples of the fluorinated saturated cyclic carbonate (A) include the following.

Those represented by the following formulas:

may be mentioned as specific examples of the fluorinated saturated cyclic carbonate (A) represented by the formula (A) in which at least one of X¹ to X⁴ is —F. These compounds have a high withstand voltage and give a good solubility of the electrolyte salt.

Alternatively, those represented by the following formula:

may also be used.

Those represented by the following formulas:

may be mentioned as specific examples of the fluorinated saturated cyclic carbonate (A) represented by the formula (A) in which at least one of X¹ to X⁴ is a fluorinated alkyl group (a) and the others thereof are —H.

Those represented by the following formulas:

may be mentioned as specific examples of the fluorinated saturated cyclic carbonate (A) represented by the formula (A) in which at least one of X¹ to X⁴ is a fluorinated alkyl group (b) having an ether bond or a fluorinated alkoxy group (c) and the others thereof are —H.

The fluorinated saturated cyclic carbonate (A) is not limited to the above specific examples. One of the above fluorinated saturated cyclic carbonates (A) may be used alone, or two or more thereof may be used in any combination at any ratio.

Preferred as the fluorinated saturated cyclic carbonate (A) are fluoroethylene carbonate and difluoroethylene carbonate.

Examples of the non-fluorinated acyclic carbonate include hydrocarbon acyclic carbonates such as CH₃OCOOCH₃ (dimethyl carbonate: DMC), CH₃CH₂OCOOCH₂CH₃ (diethyl carbonate: DEC), CH₃CH₂OCOOCH₃ (ethyl methyl carbonate: EMC), CH₃OCOOCH₂CH₂CH₃ (methyl propyl carbonate), methyl butyl carbonate, ethyl propyl carbonate, and ethyl butyl carbonate. Preferred among these is at least one compound selected from the group consisting of dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, methyl propyl carbonate, methyl butyl carbonate, ethyl propyl carbonate, and ethyl butyl carbonate.

In the electrolyte solution of the present invention, the total amount of the non-fluorinated saturated cyclic carbonate, the fluorinated saturated cyclic carbonate, and the non-fluorinated acyclic carbonate is preferably 10 to 95 mass %, more preferably 15 to 80 mass %.

A preferred aspect of the electrolyte solution of the present invention is that the electrolyte solution (hereinafter, referred to as a first electrolyte solution) contains 0.001 to less than 20 mass % of the fluorine-containing acyclic carbonate represented by the formula (1) and 0.001 to 20 mass % of the fluorine-containing succinic anhydride represented by the formula (2). The electrolyte solution containing a relatively small amount of the fluorine-containing acyclic carbonate allows an electrochemical device such as a secondary battery including the electrolyte solution to be less likely to generate gas and have a high storage capacity retention ratio.

The amount of the fluorine-containing acyclic carbonate in the first electrolyte solution is preferably 0.01 mass % or more, more preferably 0.1 mass % or more, while preferably 8 mass % or less, more preferably 6 mass % or less.

The amount of the fluorine-containing succinic anhydride in the first electrolyte solution is preferably 0.01 mass % or more, more preferably 0.1 mass % or more, while preferably 8 mass % or less, more preferably 6 mass % or less.

The first electrolyte solution preferably further contains at least one selected from the group consisting of a non-fluorinated saturated cyclic carbonate and a non-fluorinated acyclic carbonate, and more preferably contains a non-fluorinated saturated cyclic carbonate and a non-fluorinated acyclic carbonate.

The non-fluorinated saturated cyclic carbonate and the non-fluorinated acyclic carbonate may be any of those mentioned above, and are preferably any of those mentioned as preferred examples. More preferably, the first electrolyte solution contains at least one non-fluorinated saturated cyclic carbonate selected from the group consisting of ethylene carbonate, propylene carbonate, and butylene carbonate; and at least one non-fluorinated acyclic carbonate selected from the group consisting of dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, methyl propyl carbonate, methyl butyl carbonate, ethyl propyl carbonate, and ethyl butyl carbonate.

The total amount of the non-fluorinated saturated cyclic carbonate and the non-fluorinated acyclic carbonate in the first electrolyte solution is preferably 10 to 95 vol %, more preferably 15 vol % or more, still more preferably 20 vol % or more, while more preferably 80 vol % or less, still more preferably 75 vol % or less.

In the first electrolyte solution, the amount of the fluorine-containing acyclic carbonate is preferably 0.001 to 20 mass % relative to the total amount of the non-fluorinated saturated cyclic carbonate and the non-fluorinated acyclic carbonate. Too large or too small an amount of the fluorine-containing acyclic carbonate fails to suppress generation of gas and to improve the storage characteristics. The amount of the fluorine-containing acyclic carbonate is preferably 0.01 mass % or more, more preferably 0.1 mass % or more, while preferably 8 mass % or less, more preferably 6 mass % or less.

In the first electrolyte solution, the amount of the fluorine-containing succinic anhydride is 0.001 to 20 mass % relative to the total amount of the non-fluorinated saturated cyclic carbonate and the non-fluorinated acyclic carbonate. Too large or too small an amount of the fluorine-containing succinic anhydride fails to suppress generation of gas and to improve the storage characteristics. The amount of the fluorine-containing succinic anhydride is preferably 0.01 mass % or more, more preferably 0.1 mass % or more, while preferably 8 mass % or less, more preferably 6 mass % or less.

Another preferred aspect of the electrolyte solution of the present invention is that the electrolyte solution (hereinafter, referred to as a second electrolyte solution) contains not less than 20 vol % but less than 90 vol % of the fluorine-containing acyclic carbonate represented by the formula (1) and 0.001 to 20 mass % of the fluorine-containing succinic anhydride represented by the formula (2). The electrolyte solution containing a relatively large amount of the fluorine-containing acyclic carbonate allows an electrochemical device such as a secondary battery including the electrolyte solution to be less likely to generate gas and have a high storage capacity retention ratio.

The second electrolyte solution contains not less than 20 vol % but less than 90 vol % of the fluorine-containing acyclic carbonate. Too large or too small an amount of the fluorine-containing acyclic carbonate fails to suppress generation of gas and to improve the storage capacity retention ratio. The amount of the fluorine-containing acyclic carbonate is more preferably 25 vol % or more, still more preferably 30 vol % or more. The amount thereof is more preferably 85 vol % or less, still more preferably 80 vol % or less, even more preferably 75 vol % or less.

The second electrolyte solution preferably further contains a fluorinated saturated cyclic carbonate.

The fluorinated saturated cyclic carbonate may be any of those mentioned above, and is preferably any of those mentioned as preferred examples. The second electrolyte solution preferably contains at least one fluorinated saturated cyclic carbonate selected from the group consisting of fluoroethylene carbonate and difluoroethylene carbonate. In the second electrolyte solution, the mass ratio between the fluorine-containing acyclic carbonate represented by the formula (1) and the fluorinated saturated cyclic carbonate is preferably 15/85 to 85/15.

In the second electrolyte solution, the amount of the fluorine-containing succinic anhydride is preferably 0.001 to 20 mass % relative to the total amount of the fluorine-containing acyclic carbonate and the fluorinated saturated cyclic carbonate. Too large or too small an amount of the fluorine-containing succinic anhydride fails to suppress generation of gas and to improve the storage capacity retention ratio. The amount of the fluorine-containing succinic anhydride is more preferably 0.01 mass % or more, still more preferably 0.1 mass % or more, while more preferably 8 mass % or less, still more preferably 6 mass % or less.

The electrolyte solution of the present invention (including the first electrolyte solution and the second electrolyte solution) contains an electrolyte salt.

Any electrolyte salt usable for electrolyte solutions for electrochemical devices such as secondary batteries and electric double-layer capacitors may be used. Preferred is a lithium salt.

Examples of the lithium salt include inorganic lithium salts such as LiClO₄, LiPF₆, and LiBF₄; and fluoroorganic acid lithium salts such as LiSO₃CF₃, LiN(SO₂CF₃)₂, LiN(SO₂C₂F₅)₂, LiN(SO₂CF₃) (SO₂C₄F₉), LiC(SO₂CF₃)₃, LiPF₄ (CF₃)₂, LiPF₄ (C₂F₅)₂, LiPF₄(SO₂CF₃)₂, LiPF₄ (SO₂C₂F₅)₂, LiBF₂ (CF₃)₂, LiBF₂ (C₂F₅)₂, LiBF₂ (SO₂CF₃)₂, LiBF₂ (SO₂C₂F₅)₂, lithium difluoro(oxalato)borate, lithium bis(oxalato)borate, and salts represented by the formula: LiPF_(a)(C_(n)F_(2n+1))_(6-a) (wherein a is an integer of 0 to 5; and n is an integer of 1 to 6). These may be used alone or in combination of two or more.

In order to suppress degradation of the electrolyte solution after high-temperature storage, the lithium salt is particularly preferably at least one selected from the group consisting of LiPF₆, LiBF₄, LiSO₃CF₃, LiN(SO₂CF₃)₂, LiN(SO₂C₂F₅)₂, lithium difluoro(oxalato)borate, lithium bis(oxalato)borate, and salts represented by the formula: LiPF_(a)(C_(n)F_(2n+1))_(6-a) (wherein a is an integer of 0 to 5; and n is an integer of 1 to 6).

Examples of the salts represented by the formula: LiPF_(a)(C_(n)F_(2n+1))_(6-a) include LiPF₃(CF₃)₃, LiPF₃(C₂F₅)₃, LiPF₃(C₃F₇)₃, LiPF₃ (C₄F₉)₃, LiPF₄(CF₃)₂, LiPF₄(C₂F₅)₂, LiPF₄(C₃F₇)₂, and LiPF₄(C₄F₉)₂ wherein the alkyl group represented by C₃F₇ or C₄F₉ in the formula may be either linear or branched.

The concentration of the electrolyte salt in the electrolyte solution is preferably 0.5 to 3 mol/L. If the concentration thereof is outside this range, the electrolyte solution tends to have a low electric conductivity and the battery performance tends to be deteriorated.

The concentration of the electrolyte salt is more preferably 0.9 mol/L or more and 1.5 mol/L or less.

The electrolyte salt is preferably an ammonium salt.

Examples of the ammonium salt include the following salts (IIa) to (IIe).

(IIa) Tetraalkyl Quaternary Ammonium Salts

Preferred examples thereof include tetraalkyl quaternary ammonium salts represented by the following formula (IIa):

(wherein R^(1a), R^(2a), R^(3a), and R^(4a) may be the same as or different from each other, and individually represent a C1-C6 alkyl group which may optionally have an ether bond; and X⁻ represents an anion). In order to improve the oxidation resistance, part or all of the hydrogen atoms in the ammonium salt is also preferably replaced by a fluorine atom and/or a C1-C4 fluorine-containing alkyl group.

Preferred specific examples of tetraalkyl quaternary ammonium salt include tetraalkyl quaternary ammonium salts represented by the following formula (IIa-1):

(wherein R^(1a), R^(2a), and X⁻ are defined in the same manner as mentioned above; x and y may be the same as or different from each other, and individually represent an integer of 0 to 4, where x+y=4), and alkyl ether group-containing trialkyl ammonium salts represented by the following formula (IIa-2):

(wherein R^(5a) represents a C1-C6 alkyl group; R^(6a) represents a C1-C6 divalent hydrocarbon group; R^(7a) represents a C1-C4 alkyl group; z is 1 or 2; and X⁻ represents an anion). Introduction of an alkyl ether group may lead to reduction in the viscosity.

The anion X⁻ may either an inorganic anion or an organic anion. Examples of the inorganic anion include AlCl₄ ⁻, BF₄ ⁻, PF₆ ⁻, AsF₆ ⁻, TaF₆ ⁻, I⁻, and SbF₆ ⁻. Examples of the organic anion include CF₃COO⁻, CF₃SO₃ ⁻, (CF₃SO₂)₂N⁻, and (C₂F₅SO₂)₂N⁻.

In order to achieve good oxidation resistance and ionic dissociation, BF₄ ⁻, PF₆ ⁻, AsF₆ ⁻, and SbF₆ ⁻ are preferred.

Preferred specific examples of the tetraalkyl quaternary ammonium salt include Et₄NBF₄, Et₄NClO₄, Et₄NPF₆, Et₄NAsF₆, Et₄NSbF₆, Et₄NCF₃SO₃, Et₄N(CF₃SO₂)₂N, Et₄NC₄F₉SO₃, Et₃MeNBF₄, Et₃MeNClO₄, Et₃MeNPF₆, Et₃MeNAsF₆, Et₃MeNSbF₆, Et₃MeNCF₃SO₃, Et₃MeN(CF₃SO₂)₂N, Et₃MeNC₄F₉SO₃, and N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium salt. Particularly preferred examples thereof include Et₄NBF₄, Et₄NPF₆, Et₄NSbF₆, Et₄NAsF₆, Et₃MeNBF₄, and an N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium salt.

(IIb) Spirocyclic Bipyrrolidinium Salts

Preferred examples thereof include spirocyclic bipyrrolidinium salts represented by the following formula (IIb-1):

(wherein R^(8a) and R^(9a) may be the same as or different from each other, and individually represent a C1-C4 alkyl group; X⁻ represents an anion; n1 is an integer of 0 to 5; and n2 is an integer of 0 to 5); spirocyclic bipyrrolidinium salts represented by the following formula (IIb-2):

(wherein R^(10a) and R^(11a) may be the same as or different from each other, and individually represent a C1-C4 alkyl group; X⁻ represents an anion; n3 is an integer of 0 to 5; and n4 is an integer of 0 to 5); and spirocyclic bipyrrolidinium salts represented by the following formula (IIb-3):

(wherein R^(12a) and R^(13a) may be the same as or different from each other, and individually represent a C1-C4 alkyl group; X⁻ represents an anion; n5 is an integer of 0 to 5; and n6 is an integer of 0 to 5). In order to improve the oxidation resistance, part or all of the hydrogen atoms in the spirocyclic bipyrrolidinium salt is also preferably replaced by a fluorine atom and/or a C1-C4 fluorine-containing alkyl group.

Preferred specific examples of the anion X⁻ are the same as those mentioned for the salts (IIa). In order to achieve good dissociation and a low internal resistance under high voltage, BF₄ ⁻, PF₅ ⁻, (CF₃SO₂)₂N⁻, or (C₂F₅SO₂)₂N is particularly preferred.

For example, those represented by the following formulas:

may be mentioned as preferred specific examples of the spirocyclic bipyrrolidinium salts.

These spirocyclic bipyrrolidinium salts are excellent in the solubility in a solvent, the oxidation resistance, and the ion conductivity.

(IIc) Imidazolium Salts

Preferred examples thereof include imidazolium salts represented by the following formula (IIc):

(wherein R^(14a) and R^(15a) may be the same as or different from each other, and individually represent a C1-C6 alkyl group; and X⁻ represents an anion). In order to improve the oxidation resistance, part or all of the hydrogen atoms in the imidazolium salt is also preferably replaced by a fluorine atom and/or a C1-C4 fluorine-containing alkyl group.

Preferred specific examples of the anion X⁻ are the same as those mentioned for the salts (IIa).

For example, one represented by the following formula:

may be mentioned as a preferred specific example of the imidazolium salt.

This imidazolium salt is excellent in that it has a low viscosity and a good solubility.

(IId): N-Alkylpyridinium Salts

Preferred examples thereof include N-alkylpyridinium salts represented by the formula (IId):

(wherein R^(16a) represents a C1-C6 alkyl group; and X⁻ represents an anion). In order to improve the oxidation resistance, part or all of the hydrogen atoms in the N-alkylpyridinium salt is also preferably replaced by a fluorine atom and/or a C1-C4 fluorine-containing alkyl group.

Preferred specific examples of the anion X⁻ are the same as those mentioned for the salts (IIa).

For example, those represented by the following formulas:

may be mentioned as preferred specific examples of the N-alkylpyridinium salt.

These N-alkylpyridinium salts are excellent in that they have a low viscosity and a good solubility.

(IIe) N,N-Dialkylpyrrolidinium Salts

Preferred examples thereof include N,N-dialkylpyrrolidinium salts represented by the formula (IIe):

(wherein R^(17a) and R^(18a) may be the same as or different from each other, and individually represent a C1-C6 alkyl group; and X⁻ represents an anion.) In order to improve the oxidation resistance, part or all of the hydrogen atoms in the N,N-dialkylpyrrolidinium salt is also preferably replaced by a fluorine atom and/or a C1-C4 fluorine-containing alkyl group.

Preferred specific examples of the anion X⁻ are the same as those mentioned for the salts (IIa).

For example, those represented by the following formulas:

may be mentioned as preferred specific examples of the N,N-dialkylpyrrolidinium salt.

These N,N-dialkylpyrrolidinium salts are excellent in that they have a low viscosity and a good solubility.

Preferred among these ammonium salts are those represented by the formula (IIa), (IIb), or (IIc) because they have a good solubility, oxidation resistance, and ion conductivity. More preferred are those represented by any of the formulas:

wherein Me represents a methyl group; Et represents an ethyl group; X⁻, x, and y are defined in the same manner as in the formula (IIa-1).

The electrolyte salt may be a lithium salt. Preferred examples of the lithium salt include LiPF₆, LiBF₄, LiAsF₆, LiSbF₆, and LiN(SO₂C₂H₅)₂.

In order to further increase the capacity, a magnesium salt may be used. Preferred examples of the magnesium salt include Mg(ClO₄)₂ and Mg(OOC₂H₅)₂.

If the electrolyte salt is any of the above ammonium salts, the concentration thereof is preferably 0.6 mol/L or higher. If the concentration thereof is lower than 0.6 mol/L, not only the low-temperature characteristics may be poor but also the initial internal resistance may be high. The concentration of the electrolyte salt is more preferably 0.9 mol/L or higher.

For good low-temperature characteristics, the concentration is preferably 3.0 mol/L or lower, more preferably 2.0 mol/L or lower.

If the ammonium salt is triethyl methyl ammonium tetrafluoroborate (TEMABF₄), the concentration thereof is preferably 0.8 to 1.9 mol/L in order to achieve excellent low-temperature characteristics.

If the ammonium salt is spirobipyrrolidinium tetrafluoroborate (SBPBF₄), the concentration thereof is preferably 0.7 to 2.0 mol/L.

The electrolyte solution of the present invention preferably further comprises polyethylene oxide that has a weight average molecular weight of 2000 to 4000 and has —OH, —OCOOH, or —COOH at an end.

Containing such a compound improves the stability at the interfaces between the electrolyte solution and the respective electrodes, and thus can improve the battery characteristics.

Examples of the polyethylene oxide include polyethylene oxide monool, polyethylene oxide carboxylate, polyethylene oxide diol, polyethylene oxide dicarboxylate, polyethylene oxide triol, and polyethylene oxide tricarboxylate. These may be used alone or in combination of two or more.

In order to achieve good battery characteristics, a mixture of polyethylene oxide monool and polyethylene oxide diol and a mixture of polyethylene oxide carboxylate and polyethylene oxide dicarboxylate are preferred.

The polyethylene oxide having too small a weight average molecular weight may be easily oxidatively decomposed. The weight average molecular weight is more preferably 3000 to 4000.

The weight average molecular weight can be determined in terms of polystyrene equivalent by gel permeation chromatography (GPC).

The amount of the polyethylene oxide is preferably 1×10⁻⁶ to 1×10⁻² mol/kg in the electrolyte solution. If the amount of the polyethylene oxide is too large, the battery characteristics may be poor.

The amount of the polyethylene oxide is more preferably 5×10⁻⁶ mol/kg or more.

The electrolyte solution of the present invention preferably further contains, as an additive, at least one selected from the group consisting of unsaturated cyclic carbonates, fluorinated saturated cyclic carbonates, and cyclic sulfonic acid compounds. Containing any of these compounds suppresses degradation of the battery characteristics.

The unsaturated cyclic carbonate is a cyclic carbonate having an unsaturated bond, i.e., a cyclic carbonate having at least one carbon-carbon unsaturated bond in the molecule. Specific examples thereof include vinylene carbonate compounds such as vinylene carbonate, methyl vinylene carbonate, ethyl vinylene carbonate, 4,5-dimethyl vinylene carbonate, and 4,5-diethyl vinylene carbonate; and vinyl ethylene carbonate compounds such as 4-vinyl ethylene carbonate (VEC), 4-methyl-4-vinyl ethylene carbonate, 4-ethyl-4-vinyl ethylene carbonate, 4-n-propyl-4-vinyl ethylene carbonate, 5-methyl-4-vinyl ethylene carbonate, 4,4-divinyl ethylene carbonate, 4,5-divinyl ethylene carbonate, 4,4-dimethyl-5-methylene ethylene carbonate, and 4,4-diethyl-5-methylene ethylene carbonate. Preferred among these is vinylene carbonate, 4-vinyl ethylene carbonate, 4-methyl-4-vinyl ethylene carbonate, or 4,5-divinyl ethylene carbonate, and particularly preferred is vinylene carbonate or 4-vinyl ethylene carbonate.

The unsaturated cyclic carbonate may have any molecular weight that does not significantly deteriorate the effects of the present invention. The molecular weight is preferably 50 or higher and 250 or lower. The unsaturated cyclic carbonate having a molecular weight within this range is likely to assure its solubility in the electrolyte solution and to enable sufficient achievement of the effects of the present invention. The molecular weight of the unsaturated cyclic carbonate is more preferably 80 or higher, more preferably 150 or lower.

The unsaturated cyclic carbonate may be suitably a fluorinated unsaturated cyclic carbonate.

The number of fluorine atoms in the fluorinated unsaturated cyclic carbonate may be any number that is 1 or greater. The number of fluorine atoms is usually 6 or smaller, preferably 4 or smaller, most preferably 1 or 2.

Examples of the fluorinated unsaturated cyclic carbonate include fluorinated vinylene carbonate derivatives and fluorinated ethylene carbonate derivatives substituted with a substituent having an aromatic ring or a carbon-carbon double bond.

Examples of the fluorinated vinylene carbonate derivatives include 4-fluorovinylene carbonate, 4-fluoro-5-methyl vinylene carbonate, 4-fluoro-5-phenyl vinylene carbonate, 4-allyl-5-fluorovinylene carbonate, and 4-fluoro-5-vinyl vinylene carbonate.

Examples of the fluorinated ethylene carbonate derivatives substituted with a substituent having an aromatic ring or a carbon-carbon double bond include 4-fluoro-4-vinyl ethylene carbonate, 4-fluoro-4-allyl ethylene carbonate, 4-fluoro-5-vinyl ethylene carbonate, 4-fluoro-5-allyl ethylene carbonate, 4,4-difluoro-4-vinyl ethylene carbonate, 4,4-difluoro-4-allyl ethylene carbonate, 4,5-difluoro-4-vinyl ethylene carbonate, 4,5-difluoro-4-allyl ethylene carbonate, 4-fluoro-4,5-divinyl ethylene carbonate, 4-fluoro-4,5-diallyl ethylene carbonate, 4,5-difluoro-4,5-divinyl ethylene carbonate, 4,5-difluoro-4,5-diallyl ethylene carbonate, 4-fluoro-4-phenyl ethylene carbonate, 4-fluoro-5-phenyl ethylene carbonate, 4,4-difluoro-5-phenyl ethylene carbonate, and 4,5-difluoro-4-phenyl ethylene carbonate.

The fluorinated unsaturated cyclic carbonate may have any molecular weight that does not significantly deteriorate the effects of the present invention. The molecular weight is preferably 50 or higher and 500 or lower. The fluorinated unsaturated cyclic carbonate having a molecular weight within this range is likely to assure the solubility of the fluorinated unsaturated cyclic carbonate in the electrolyte solution and to exert the effects of the present invention.

The unsaturated cyclic carbonates may be used alone or in any combination of two or more at any ratio.

Examples of the fluorinated saturated cyclic carbonate include compounds mentioned as examples of the fluorinated saturated cyclic carbonates usable for the solvent.

Examples of the cyclic sulfonic acid compounds include 1,3-propane sultone, 1,4-butane sultone, 1-fluoro-1,3-propane sultone, 2-fluoro-1,3-propane sultone, and 3-fluoro-1,3-propane sultone.

In order to improve the high-temperature characteristics, the electrolyte solution of the present invention preferably contains 1,3-propane sultone and/or 1,4-butane sultone.

If at least one compound selected from the group consisting of the unsaturated cyclic carbonates, fluorinated saturated cyclic carbonates, and the cyclic sulfonic acid compounds is used as an additive, the amount thereof in the electrolyte solution is preferably 0.1 to 10 mass %, more preferably 1 mass % or more, while more preferably 5 mass % or less.

The electrolyte solution of the present invention may further contain any other solvents or additives such as cyclic or acyclic carboxylate esters, ether compounds, nitrogen-containing compounds, boron-containing compounds, organic silicon-containing compounds, fireproof agents (flame retardants), surfactants, additives for increasing the permittivity, improvers for cycle characteristics and rate characteristics, and overcharge inhibitors, to the extent that the effects of the present invention are not deteriorated.

Examples of the cyclic carboxylate esters include those having 3 to 12 carbon atoms in total in the structural formula. Specific examples thereof include gamma-butyrolactone, gamma-valerolactone, gamma-caprolactone, and epsilon-caprolactone. Particularly preferred is gamma-butyrolactone because it can improve the battery characteristics owing to improvement in the degree of dissociation of lithium ions.

In general, the amount of the cyclic carboxylate ester is preferably 0.1 mass % or more, more preferably 1 mass % or more, in 100 mass % of the solvent. The cyclic carboxylate ester in an amount within this range is likely to improve the electric conductivity of the electrolyte solution, and thus to improve the large-current discharge characteristics of electrolyte batteries. The amount of the cyclic carboxylate ester is also preferably 10 mass % or less, more preferably 5 mass % or less. Such an upper limit may allow the electrolyte solution to have a viscosity within an appropriate range, may make it possible to avoid a reduction in the electric conductivity, may suppress an increase in the resistance of the negative electrode, and may allow electrolyte batteries to have large-current discharge characteristics within a favorable range.

The cyclic carboxylate ester to be suitably used may be a fluorinated cyclic carboxylate ester (fluorolactone). Examples of the fluorolactone include fluorolactones represented by the following formula (C):

wherein X¹⁵ to X²⁰ may be the same as or different from each other, and individually represent —H, —F, —Cl, —CH₃, or a fluorinated alkyl group; and at least one of X¹⁵ to X²⁰ is a fluorinated alkyl group.

Examples of the fluorinated alkyl group for X¹⁵ to X²⁰ include —CFH₂, —CF₂H, —CF₃, —CH₂CF₃, —CF₂CF₃, —CH₂CF₂CF₃, and —CF(CF₃)₂. In order to achieve high oxidation resistance and an effect of improving the safety, —CH₂CF₃ and —CH₂CF₂CF₃ are preferred.

One of X¹⁵ to X²⁰ or a plurality thereof may be replaced by —H, —F, —Cl, —CH₃ or a fluorinated alkyl group only when at least one of X¹⁵ to X²⁰ is a fluorinated alkyl group. In order to achieve a good solubility of the electrolyte salt, the number of substituents is preferably 1 to 3, more preferably 1 or 2.

The substitution may be at any of the above sites in the fluorinated alkyl group. In order to achieve a good synthesizing yield, the substitution site is preferably X¹⁷ and/or X¹⁸. In particular, X¹⁷ or X¹⁸ is preferably a fluorinated alkyl group, especially, —CH₂CF₃ or —CH₂CF₂CF₃. The substituent for X¹⁵ to X²⁰ other than the fluorinated alkyl group is —H, —F, —Cl, or CH₃. In order to achieve a good solubility of the electrolyte salt, —H is preferred.

In addition to those represented by the above formula, the fluorolactone may also be a fluorolactone represented by the following formula (D):

wherein one of A and B is CX²⁶X²⁷ (where X²⁶ and X²⁷ may be the same as or different from each other, and individually represent —H, —F, —Cl, —CF₃, —CH₃, or an alkylene group in which a hydrogen atom may optionally be replaced by a halogen atom and which may optionally has a hetero atom in the chain) and the other is an oxygen atom; Rf¹² is a fluorinated alkyl group or fluorinated alkoxy group which may optionally have an ether bond; X²¹ and X²² may be the same as or different from each other, and individually represent —H, —F, —Cl, —CF₃, or CH₃; X²³ to X²⁵ may be the same as or different from each other, and individually represent —H, —F, —Cl, or an alkyl group in which a hydrogen atom may optionally be replaced by a halogen atom and which may optionally contain a hetero atom in the chain; and n=0 or 1.

Preferred examples of the fluorolactone represented by the formula (D) include a 5-membered ring structure represented by the following formula (E):

(wherein A, B, Rf¹², X²¹, X²², and X²³ are defined in the same manner as in the formula (D)) because it is easily synthesized and has good chemical stability. Further, in relation to the combination of A and B, fluorolactones represented by the following formula (F):

(wherein Rf¹², X²¹, X²², X²³, X²⁶, and X²⁷ are defined in the same manner as in the formula (D)) and fluorolactones represented by the following formula (G):

(wherein Rf¹², X²¹, X²², X²³, X²⁶, and X²⁷ are defined in the same manner as in the formula (D)) may be mentioned.

In order to particularly achieve excellent characteristics such as a high permittivity and a high withstand voltage, and to improve the characteristics of the electrolyte solution in the present invention, for example, to achieve a good solubility of the electrolyte salt and to well reduce the internal resistance, those represented by the following formulas:

may be mentioned.

Containing a fluorinated cyclic carboxylate ester leads to effects of, for example, improving the ion conductivity, improving the safety, and improving the stability at high temperature.

Examples of the acyclic carboxylate ester include those having three to seven carbon atoms in total in the structural formula. Specific examples thereof include methyl acetate, ethyl acetate, n-propyl acetate, isopropyl acetate, n-butyl acetate, isobutyl acetate, t-butyl acetate, methyl propionate, ethyl propionate, n-propyl propionate, isopropyl propionate, n-butyl propionate, isobutyl propionate, t-butyl propionate, methyl butyrate, ethyl butyrate, n-propyl butyrate, isopropyl butyrate, methyl isobutyrate, ethyl isobutyrate, n-propyl isobutyrate, and isopropyl isobutyrate.

In order to improve the ion conductivity owing to a reduction in the viscosity, preferred are methyl acetate, ethyl acetate, n-propyl acetate, n-butyl acetate, methyl propionate, ethyl propionate, n-propyl propionate, isopropyl propionate, methyl butyrate, and ethyl butyrate, for example.

Also, a fluorinated acyclic carboxylate ester may also suitably be used. Preferred examples of the fluorine-containing ester include fluorinated acyclic carboxylate esters represented by the following formula (H):

Rf¹⁰COORf¹¹  (H)

(wherein Rf¹⁰ represents a C1-C2 fluorinated alkyl group; and Rf¹¹ represents a C1-C4 fluorinated alkyl group) because they are high in flame retardance and have good compatibility with other solvents and good oxidation resistance.

Examples of the group for Rf¹⁰ include CF₃—, CF₃CF₂—, HCF₂CF₂—, HCF₂—, CH₃CF₂—, and CF₃CH₂—. In order to achieve good rate characteristics, CF₃— and CF₃CF₂— are particularly preferred.

Examples of the group for Rf¹¹ include —CF₃, —CF₂CF₃, —CH(CF₃)₂, —CH₂CF₃, —CH₂CH₂CF₃, —CH₂CF₂CFHCF₃, —CH₂C₂F₅, —CH₂CF₂CF₂H, —CH₂CH₂C₂F₅, —CH₂CF₂CF₃, and —CH₂CF₂CF₂CF₃. In order to achieve good compatibility with other solvents, —CH₂CF₃, —CH(CF₃)₂, —CH₂C₂F₅, and —CH₂CF₂CF₂H are particularly preferred.

Specifically, for example, the fluorinated acyclic carboxylate ester may comprise one or two or more of CF₃C(═O)OCH₂CF₃, CF₃C(═O)OCH₂CH₂CF₃, CF₃C(═O)OCH₂C₂F₅, CF₃C(═O)OCH₂CF₂CF₂H, and CF₃C(═O)OCH(CF₃)₂. In order to achieve good compatibility with other solvents and good rate characteristics, CF₃C(═O)OCH₂C₂F₅, CF₃C(═O)OCH₂CF₂CF₂H, CF₃C(═O)OCH₂CF₃, and CF₃C(═O)OCH(CF₃)₂ are particularly preferred.

The ether compound is preferably a C3-C10 acyclic ether or a C3-C6 cyclic ether.

Examples of the C3-C10 acyclic ether include diethyl ether, di-n-butyl ether, dimethoxy methane, methoxy ethoxy methane, diethoxy methane, dimethoxy ethane, methoxy ethoxy ethane, diethoxy ethane, ethylene glycol di-n-propyl ether, ethylene glycol di-n-butyl ether, and diethylene glycol dimethyl ether.

The ether compound may suitably be a fluorinated ether.

One example of the fluorinated ether is a fluorinated ether (I) represented by the following formula (I):

Rf¹³—O—Rf¹⁴  (I)

(wherein Rf¹³ and Rf¹⁴ may be the same as or different from each other, and individually represent a C1-C10 alkyl group or a C1-C10 fluorinated alkyl group; and at least one of Rf¹³ and Rf¹⁴ is a fluorinated alkyl group). Containing the fluorinated ether (I) can improve the flame retardance of the electrolyte solution, as well as improve the stability and safety at high temperature under high voltage.

In the formula (I), at least one of Rf¹³ and Rf¹⁴ has only to be a C1-C10 fluorinated alkyl group. In order to further improve the flame retardance and the stability and safety at high temperature under high voltage of the electrolyte solution, both Rf¹³ and Rf¹⁴ are preferably a C1-C10 fluorinated alkyl group. In this case, Rf¹³ and Rf¹⁴ may be the same as or different from each other.

Particularly preferably, Rf¹³ and Rf¹⁴ are the same as or different from each other, and Rf¹³ is a C3-C6 fluorinated alkyl group and Rf¹⁴ is a C2-C6 fluorinated alkyl group.

If the sum of the carbon numbers of Rf¹³ and Rf¹⁴ is too small, the fluorinated ether may have too low a boiling point. If the carbon number of Rf¹³ or Rf¹⁴ is too large, the solubility of the electrolyte salt may be low, which may cause a bad influence on the compatibility with other solvent, and the viscosity may be high so that the rate characteristics may be poor. In order to achieve excellent rate characteristics and boiling point, advantageously, the carbon number of Rf¹³ is 3 or 4 and the carbon number of Rf¹⁴ is 2 or 3.

The fluorinated ether (I) preferably has a fluorine content of 40 to 75 mass %. The fluorinated ether (I) having a fluorine content within this range may lead to particularly excellent balance between the flame retardance and the compatibility. The above range is also preferred for good oxidation resistance and safety.

The lower limit of the fluorine content is more preferably 45 mass %, still more preferably 50 mass %, particularly preferably 55 mass %. The upper limit thereof is more preferably 70 mass %, still more preferably 66 mass %.

The fluorine content of the fluorinated ether (I) is a value calculated based on the structural formula of the fluorinated ether (I) by the following formula:

{(number of fluorine atoms×19)/(molecular weight of fluorinated ether (I))}×100(%).

Examples of the group for Rf¹³ include CF₃CF₂CH₂—, CF₃CFHCF₂—, HCF₂CF₂CF₂—, HCF₂CF₂CH₂—, CF₃CF₂CH₂CH₂—, CF₃CFHCF₂CH₂—, HCF₂CF₂CF₂CF₂—, HCF₂CF₂CF₂CH₂—, HCF₂CF₂CH₂CH₂—, and HCF₂CF(CF₃)CH₂—. Examples of the group for Rf¹⁴ include —CH₂CF₂CF₃, —CF₂CFHCF₃, —CF₂CF₂CF₂H, —CH₂CF₂CF₂H, —CH₂CH₂CF₂CF₃, —CH₂CF₂CFHCF₃, —CF₂CF₂CF₂CF₂H, —CH₂CF₂CF₂CF₂H, —CH₂CH₂CF₂CF₂H, —CH₂CF(CF₃)CF₂H, —CF₂CF₂H, —CH₂CF₂H, and —CF₂CH₃.

Specific examples of the fluorinated ether (I) include HCF₂CF₂CH₂OCF₂CF₂H, CF₃CF₂CH₂OCF₂CF₂C₂H, HCF₂CF₂CH₂OCF₂CFHCF₃, CF₃CF₂CH₂OCF₂CFHCF₃, C₆F₁₃OCH₃, C₆F₁₃OC₂H₅, C₈F₁₇OCH₃, C₈F₁₇OC₂H₅, CF₃CFHCF₂CH(CH₃)OCF₂CFHCF₃, HCF₂CF₂OCH(C₂H₅)₂, HCF₂CF₂OC₄H₉, HCF₂CF₂OCH₂CH(C₂H₅)₂, and HCF₂CF₂OCH₂CH(CH₃)₂.

In particular, those having HCF₂— or CF₃CFH— at one end or both ends can provide a fluorinated ether (I) excellent in polarizability and having a high boiling point. The boiling point of the fluorinated ether (I) is preferably 67° C. to 120° C. It is more preferably 80° C. or higher, still more preferably 90° C. or higher.

Such a fluorinated ether (I) may comprise one or two or more of CF₃CH₂OCF₂CFHCF₃, CF₃CF₂CH₂OCF₂CFHCF₃, HCF₂CF₂CH₂OCF₂CFHCF₃, HCF₂CF₂CH₂OCH₂CF₂CF₂H, CF₃CFHCF₂CH₂OCF₂CFHCF₃, HCF₂CF₂CH₂OCF₂CF₂H, CF₃CF₂CH₂OCF₂CF₂H, and the like.

In order to advantageously achieve a high boiling point, good compatibility with other solvents, and a good solubility of the electrolyte salt, the fluorinated ether (I) is preferably at least one selected from the group consisting of HCF₂CF₂CH₂OCF₂CFHCF₃ (boiling point: 106° C.), CF₃CF₂CH₂OCF₂CFHCF₃ (boiling point: 82° C.), HCF₂CF₂CH₂OCF₂CF₂H (boiling point: 92° C.), and CF₃CF₂CH₂OCF₂CF₂H (boiling point: 68° C.), more preferably at least one selected from the group consisting of HCF₂CF₂CH₂OCF₂CFHCF₃ (boiling point: 106° C.) and HCF₂CF₂CH₂OCF₂CF₂H (boiling point: 92° C.).

Examples of the C3-C6 cyclic ether include 1,3-dioxane, 2-methyl-1,3-dioxane, 4-methyl-1,3-dioxane, 1,4-dioxane, and fluorinated compounds thereof. Preferred are dimethoxy methane, diethoxy methane, ethoxy methoxy methane, ethylene glycol n-propyl ether, ethylene glycol di-n-butyl ether, and diethylene glycol dimethyl ether because they have a high ability to solvate with lithium ions and improve the degree of ion dissociation. Particularly preferred are dimethoxy methane, diethoxy methane, and ethoxy methoxy methane because they have a low viscosity and give a high ion conductivity.

Examples of the nitrogen-containing compound include nitrile, fluorine-containing nitrile, carboxylic acid amide, fluorine-containing carboxylic acid amide, sulfonic acid amide, and fluorine-containing sulfonic acid amide. Also, 1-methyl-2-pyrrolidinone, l-methyl-2-piperidone, 3-methyl-2-oxazilidinone, 1,3-dimethyl-2-imidazolidinone, and N-methylsuccinimide may also be used.

Examples of the boron-containing compounds include borate esters such as trimethyl borate and triethyl borate, boric acid ethers, and alkyl borates.

Examples of the organic silicon-containing compounds include (CH₃)₄—Si and (CH₃)₃—Si—Si(CH₃)₃.

Examples of the fireproof agents (flame retardants) include organophosphates and phosphazene-based compounds. Examples of the organophosphates include fluoroalkyl phosphates, non-fluoroalkyl phosphates, and aryl phosphates. Particularly preferred are fluoroalkyl phosphates because they can show a flame retardant effect even at a small amount.

Specific examples of the fluoroalkyl phosphates include fluorodialkyl phosphates disclosed in JP H11-233141 A, cyclic alkyl phosphates disclosed in JP H11-283669 A, and fluorotrialkyl phosphates.

Preferred as the fireproof agents (flame retardants) are (CH₃O)₃P═O and (CF₃CH₂O)₃P═O, for example.

The surfactant may be any of cationic surfactants, anionic surfactants, nonionic surfactants, and amphoteric surfactants. In order to achieve good cycle characteristics and rate characteristics, the surfactant is preferably one containing a fluorine atom.

Preferred examples of such a surfactant containing a fluorine atom include fluorine-containing carboxylic acid salts represented by the following formula (J):

Rf¹⁵COO⁻M⁺  (J)

(wherein Rf¹⁵ represents a C3-C10 fluorine-containing alkyl group which may optionally have an ether bond; M⁺ represents Li⁺, Na⁺, K⁺, or NHR′₃ ⁺ (where R's may be the same as or different from each other, and individually represent H or a C1-C3 alkyl group)), and fluorine-containing sulfonic acid salts represented by the following formula (K):

Rf¹⁶SO₃ ⁻M⁺  (K)

(wherein Rf¹⁶ represents a C3-C10 fluorine-containing alkyl group which may optionally have an ether bond; M⁺ represents Li⁺, Na⁺, K⁺, or NHR′₃ ⁺ (where R's may be the same as or different from each other, and individually represent H or a C1-C3 alkyl group)).

In order to reduce the surface tension of the electrolyte solution without deteriorating the charge and discharge cycle characteristics, the amount of the surfactant is preferably 0.01 to 2 mass % in the electrolyte solution.

Examples of the additives for increasing the permittivity include sulfolane, methyl sulfolane, γ-butyrolactone, γ-valerolactone, acetonitrile, and propionitrile.

Examples of the improvers for cycle characteristics and rate characteristics include methyl acetate, ethyl acetate, tetrahydrofuran, and 1,4-dioxane.

In order to suppress burst or combustion of batteries in case of overcharge, for example, the overcharge inhibitor is preferably an overcharge inhibitor having an aromatic ring. Examples of the overcharge inhibitor having an aromatic ring include aromatic compounds such as cyclohexyl benzene, biphenyl, alkyl biphenyl, terphenyl, partially hydrogenated terphenyl, t-butyl benzene, t-amyl benzene, diphenyl ether, benzofuran, dibenzofuran, dichloroaniline, and toluene; fluorinated aromatic compounds such as hexafluorobenzene, fluorobenzene, 2-fluorobiphenyl, o-cyclohexyl fluorobenzene, and p-cyclohexyl fluorobenzene; and fluoroanisole compounds such as 2,4-difluoroanisole, 2,5-difluoroanisole, 2,6-difluoroanisole, and 3,5-difluoroanisole. Preferred are aromatic compounds such as biphenyl, alkyl biphenyl, terphenyl, partially hydrogenated terphenyl, cyclohexyl benzene, t-butyl benzene, t-amyl benzene, diphenyl ether, and dibenzofuran. These compounds may be used alone or in combination of two or more. In the case of combination use of two or more compounds, preferred is a combination of cyclohexyl benzene and t-butyl benzene or t-amyl benzene, or a combination of at least one oxygen-free aromatic compound selected from biphenyl, alkyl biphenyl, terphenyl, partially hydrogenated terphenyl, cyclohexyl benzene, t-butyl benzene, t-amyl benzene, and the like, and at least one oxygen-containing aromatic compound selected from diphenyl ether, dibenzofuran, and the like. Such combinations lead to good balance between the overcharge inhibiting characteristics and the high-temperature storage characteristics.

In order to suppress burst or combustion of batteries in case of overcharge, for example, the amount of the overcharge inhibitor is preferably 0.1 to 5 mass % in the electrolyte solution.

The electrolyte solution of the present invention may further contain any other known assistants to the extent that the effects of the present invention are not deteriorated. Examples of such known assistants include carbonate compounds such as erythritan carbonate, spiro-bis-dimethylene carbonate, and methoxy ethyl-methyl carbonate; carboxylic anhydrides such as succinic anhydride, glutaric anhydride, maleic anhydride, citraconic anhydride, glutaconic anhydride, itaconic anhydride, diglycolic anhydride, cyclohexanedicarboxylic anhydride, cyclopentanetetracarboxylic dianhydride, and phenylsuccinic anhydride; spiro compounds such as 2,4,8,10-tetraoxaspiro[5.5]undecane and 3,9-divinyl-2,4,8,10-tetraoxaspiro[5.5]undecane; sulfur-containing compounds such as ethylene sulfite, methyl fluorosulfonate, ethyl fluorosulfonate, methyl methanesulfonate, ethyl methanesulfonate, busulfan, sulfolene, diphenyl sulfone, N,N-dimethyl methane sulfone amide, N,N-diethyl methane sulfone amide, and like other chain sulfones, fluorine-containing chain sulfones, chain sulfonates, fluorine-containing chain sulfonates, cyclic sulfones, fluorine-containing cyclic sulfones, halides of sulfonic acid, and halides of fluorine-containing sulfonic acid; and fluoroaromatic compounds such as hydrocarbon compounds, including heptane, octane, nonane, decane, and cycloheptane. These compounds may be used alone or in combination of two or more. These assistants can improve the capacity retention characteristics and the cycle characteristics after high-temperature storage.

The electrolyte solution of the present invention may be combined with a polymer material and thereby formed into a gel-like (plasticized), gel electrolyte solution.

Examples of such a polymer material include conventionally known polyethylene oxide and polypropylene oxide, modified products thereof (see JP H08-222270 A, JP 2002-100405 A); polyacrylate-based polymers, polyacrylonitrile, and fluororesins such as polyvinylidene fluoride and vinylidene fluoride-hexafluoropropylene copolymers (see JP H04-506726 T, JP H08-507407 T, JP H10-294131 A); complexes of any of these fluororesins and any hydrocarbon resin (see JP H11-35765 A, JP H11-86630 A). In particular, polyvinylidene fluoride or a vinylidene fluoride-hexafluoropropylene copolymer is preferably used as a polymer material for gel electrolytes.

The electrolyte solution of the present invention may also contain an ion conductive compound disclosed in Japanese Patent Application No. 2004-301934.

This ion conductive compound is an amorphous fluoropolyether compound having a fluorine-containing group at a side chain and is represented by the formula (1-1):

A-(D)-B  (1-1)

wherein D is represented by the formula (2-1):

-(D1)_(n)-(FAE)_(m)-(AE)_(p)-(Y)_(q)—  (2-1)

[wherein

D1 is an ether unit having a fluoro ether group at a side chain and is represented by the formula (2a):

(wherein Rf represents a fluoro ether group which may optionally have a cross-linkable functional group; and R^(1°) represents a group or a bond that links Rf and the main chain);

FAE is an ether unit having a fluorinated alkyl group at a side chain and is represented by the formula (2b):

(wherein Rfa represents a hydrogen atom or a fluorinated alkyl group which may optionally have a cross-linkable functional group; and R¹¹ represents a group or a bond that links Rfa and the main chain);

AE is an ether unit represented by the formula (2c):

(wherein R¹³ represents a hydrogen atom, an alkyl group which may optionally have a cross-linkable functional group, an aliphatic cyclic hydrocarbon group which may optionally have a cross-linkable functional group, or an aromatic hydrocarbon group which may optionally have a cross-linkable functional group; and R¹² represents a group or a bond that links R¹³ and the main chain); Y is a unit having at least one selected from the formulas (2d-1) to (2d-3):

n is an integer of 0 to 200;

m is an integer of 0 to 200;

p is an integer of 0 to 10000;

q is an integer of 1 to 100;

n+m is not 0; and

the bonding order of D1, FAE, AE, and Y is not specified]; and

A and B may be the same as or different from each other, and individually represent a hydrogen atom, an alkyl group which may optionally have a fluorine atom and/or a cross-linkable functional group, a phenyl group which may optionally have a fluorine atom and/or a cross-linkable functional group, a —COOH group, —OR (where R represents a hydrogen atom or an alkyl group which may optionally have a fluorine atom and/or a cross-linkable functional group), an ester group, or a carbonate group (if an end of D is an oxygen atom, A and B each are none of a —COOH group, —OR, an ester group, and a carbonate group).

The electrolyte solution of the present invention may further contain any other additives, if necessary. Examples of such other additives include metal oxides and glass.

The electrolyte solution of the present invention may be prepared by any method using the aforementioned components.

As mentioned above, the electrolyte solution of the present invention comprises the specific amounts of the specific fluorine-containing compounds. Such a configuration suppresses generation of gas, and thereby enables production of a secondary battery excellent in high-temperature storage characteristics. Accordingly, the electrolyte solution of the present invention can be suitably applied to electrochemical devices such as secondary batteries. Such an electrochemical device or secondary battery comprising the electrolyte solution of the present invention is also one aspect of the present invention.

Examples of the electrochemical devices include lithium ion secondary batteries, capacitors (electric double-layer capacitors), radical batteries, solar cells (in particular, dye-sensitized solar cells), fuel cells, various electrochemical sensors, electrochromic elements, electrochemical switching elements, aluminum electrolytic capacitors, and tantalum electrolytic capacitors. Preferred are lithium ion secondary batteries and electric double-layer capacitors.

In the following, a lithium ion secondary battery is described as an example of the electrochemical device or secondary battery of the present invention.

The lithium ion secondary battery comprises a positive electrode, a negative electrode, and the aforementioned electrolyte solution.

<Positive Electrode>

The positive electrode is formed from a positive electrode mixture containing a positive electrode active material, which is a material of the positive electrode, and a current collector.

The positive electrode active material may be any material that can electrochemically occlude and release lithium ions. For example, a substance containing lithium and at least one transition metal is preferred. Specific examples thereof include lithium-containing transition metal complex oxides and lithium-containing transition metal phosphoric acid compounds. In particular, the positive electrode active material is preferably a lithium-containing transition metal complex oxide that generates a high voltage.

Examples of the lithium-containing transition metal complex oxides include

lithium-manganese spinel complex oxides represented by the formula (L): Li_(a)Mn_(2-b)M¹ _(b)O₄ (wherein 0.9≦a; 0≦b≦1.5; and M¹ is at least one metal selected from the group consisting of Fe, Co, Ni, Cu, Zn, Al, Sn, Cr, V, Ti, Mg, Ca, Sr, B, Ga, In, Si, and Ge), lithium-nickel complex oxides represented by the formula (M): LiNi_(1-c)M² _(c)O₂ (wherein 0≦c≦0.5; and M² is at least one metal selected from the group consisting of Fe, Co, Mn, Cu, Zn, Al, Sn, Cr, V, Ti, Mg, Ca, Sr, B, Ga, In, Si, and Ge), and

lithium-cobalt complex oxides represented by the formula (N): LiCo_(1-d)M³ _(d)O₂ (wherein 0≦d≦0.5; and M³ is at least one metal selected from the group consisting of Fe, Ni, Mn, Cu, Zn, Al, Sn, Cr, V, Ti, Mg, Ca, Sr, B, Ga, In, Si, and Ge).

In order to provide a high-power lithium ion secondary battery having a high energy density, preferred is LiCoO₂, LiMnO₂, LiNiO₂, LiMn₂O₄, LiNi_(0.8)Co_(0.15)Al_(0.05)O₂, or LiNi_(1/3)Co_(1/3)Mn_(1/3)O₂.

Examples of other positive electrode active materials include LiFePO₄, LiNi_(0.8)Co_(0.2)O₂, Li_(1.2)Fe_(0.4)Mn_(0.4)O₂, LiNi_(0.5)Mn_(0.5)O₂, LiV₃O₆, and LiNi_(0.5)Mn_(0.5)O₄.

In order to improve the continuous charge characteristics, the positive electrode active material preferably contains lithium phosphate. The use of lithium phosphate may be achieved in any manner, and the positive electrode active material and lithium phosphate are preferably used in admixture. The amount of lithium phosphate in the sum of the amounts of the positive electrode active material and the lithium phosphate is preferably 0.1 mass % or more, more preferably 0.3 mass % or more, still more preferably 0.5 mass % or more, whereas the amount thereof is preferably 10 mass % or less, more preferably 8 mass % or less, still more preferably 5 mass % or less.

To the surface of the positive electrode active material may be attached a substance having a composition different from the positive electrode active material. Examples of the substance attached to the surface include oxides such as aluminum oxide, silicon oxide, titanium oxide, zirconium oxide, magnesium oxide, calcium oxide, boron oxide, antimony oxide, and bismuth oxide; sulfates such as lithium sulfate, sodium sulfate, potassium sulfate, magnesium sulfate, calcium sulfate, and aluminum sulfate; carbonates such as lithium carbonate, calcium carbonate, magnesium carbonate; and carbon.

Such a substance may be attached to the surface of the positive electrode active material by, for example, a method of dissolving or suspending the substance in a solvent, impregnating the solution or suspension into the positive electrode active material, and drying the impregnated material; a method of dissolving or suspending a precursor of the substance in a solvent, impregnating the solution or suspension into the positive electrode active material, and reacting the material and the precursor by heating; or a method of adding the substance to a precursor of the positive electrode active material and simultaneously sintering the materials. In the case where carbon is to be attached, a carbonaceous material in the form of activated carbon, for example, may be mechanically attached to the surface afterward.

The amount (in terms of mass) of the substance attached to the surface relative to the amount of the positive electrode active material is preferably 0.1 ppm or more, more preferably 1 ppm or more, still more preferably 10 ppm or more, whereas the amount thereof is preferably 20% or less, more preferably 10% or less, still more preferably 5% or less. The substance attached to the surface suppresses the oxidation of the electrolyte solution on the surface of the positive electrode active material, so that the battery life is improved. If the amount thereof is too small, the substance fails to sufficiently provide the effect. If the amount thereof is too large, the substance may hinder the entrance and exit of lithium ions, so that the resistance may be increased.

Particles of the positive electrode active material may have any conventionally used shape, such as an agglomerative shape, a polyhedral shape, a spherical shape, an ellipsoidal shape, a plate shape, a needle shape, or a pillar shape. The primary particles may agglomerate to form secondary particles.

The positive electrode active material has a tap density of preferably 0.5 g/cm³ or higher, more preferably 0.8 g/cm³ or higher, still more preferably 1.0 g/cm³ or higher. If the tap density of the positive electrode active material is below the lower limit, an increased amount of a dispersion medium is required, as well as increased amounts of a conductive material and a binder are required in formation of a positive electrode active material layer. Thus, the filling rate of the positive electrode active material into the positive electrode active material layer may be limited and the battery capacity may be limited. With a complex oxide powder having a high tap density, a positive electrode active material layer with a high density can be formed. The tap density is preferably as high as possible and has no upper limit, in general. Still, if the tap density is too high, diffusion of lithium ions in the positive electrode active material layer with the electrolyte solution serving as a diffusion medium may function as a rate-determining step, so that the load characteristics may be easily deteriorated. Thus, the tap density is preferably 4.0 g/cm³ or lower, more preferably 3.7 g/cm³ or lower, still more preferably 3.5 g/cm³ or lower.

The tap density is determined as a powder filling density (tap density) g/cm³ when 5 to 10 g of the positive electrode active material powder is filled into a 10-ml glass graduated cylinder and the cylinder is tapped 200 times with a stroke of about 20 mm.

The particles of the positive electrode active material have a median size d50 (if the primary particles agglomerate to form secondary particles, the secondary particle size) of preferably 0.3 μm or greater, more preferably 0.5 μm or greater, still more preferably 0.8 μm or greater, most preferably 1.0 μm or greater, whereas the median size d50 is preferably 30 μm or smaller, more preferably 27 μm or smaller, still more preferably 25 μm or smaller, most preferably 22 μm or smaller. If the median size is below the lower limit, products with a high tap density may not be obtained. If the median size exceeds the upper limit, diffusion of lithium in the particles may take a long time, so that the battery performance may be poor or streaks may be formed in formation of positive electrodes for batteries, i.e., when the active material and components such as a conductive material and a binder are formed into slurry by adding a solvent and the slurry is applied in the form of a film, for example. Mixing two or more positive electrode active materials having different median sizes d50 leads to further improved filling in formation of positive electrodes.

The median size d50 is determined using a known laser diffraction/scattering particle size distribution analyzer. In the case of using LA-920 (Horiba, Ltd.) as the particle size distribution analyzer, the dispersion medium used in the measurement is a 0.1 mass % sodium hexametaphosphate aqueous solution and the measurement refractive index is set to 1.24 after 5-minute ultrasonic dispersion.

If the primary particles agglomerate to form secondary particles, the average primary particle size of the positive electrode active material is preferably 0.05 μm or greater, more preferably 0.1 μm or greater, still more preferably 0.2 μm or greater. The average primary particle size is preferably 5 μm or smaller, more preferably 4 μm or smaller, still more preferably 3 μm or smaller, most preferably 2 μm or smaller. If the average primary particle size exceeds the upper limit, spherical secondary particles are difficult to form, which may have a bad influence on the powder filling or may cause a great reduction in the specific surface area. Thus, the battery performance such as output characteristics is more likely to be deteriorated. In contrast, if the average primary particle size is below the lower limit, the crystals usually do not sufficiently grow. Thus, charge and discharge may be poorly reversible.

The primary particle size is measured by scanning electron microscopic (SEM) observation. Specifically, the primary particle size is determined as follows. A photograph at a magnification of 10000× is first taken. Any 50 primary particles are selected and the maximum length between the left and right boundary lines of each primary particle is measured along the horizontal line. Then, the average value of the maximum lengths is calculated, which is defined as the primary particle size.

The positive electrode active material has a BET specific surface area of preferably 0.1 m²/g or larger, more preferably 0.2 m²/g or larger, still more preferably 0.3 m²/g or larger. The BET specific surface area is preferably 50 m²/g or smaller, more preferably 40 m²/g or smaller, still more preferably 30 m²/g or smaller. If the BET specific surface area is smaller than the above range, the battery performance may be easily deteriorated. If it is larger than the above range, the tap density is less likely to be high and formation of the positive electrode active material layer may involve a difficulty in applying the material.

The BET specific surface area is defined by a value determined by single point BET nitrogen adsorption utilizing a gas flow method using a surface area analyzer (e.g., fully automatic surface area measurement device, Ohkura Riken Co., Ltd.), a sample pre-dried in the stream of nitrogen at 150° C. for 30 minutes, and a nitrogen-helium gas mixture with the nitrogen pressure relative to the atmospheric pressure being accurately adjusted to 0.3.

When the lithium ion secondary battery is used as a large-size lithium ion secondary battery for hybrid vehicles or distributed generation, it is required to achieve a high output. Thus, the particles of the positive electrode active material preferably mainly include secondary particles.

The particles of the positive electrode active material preferably include 0.5 to 7.0 vol % of fine particles having an average secondary particle size of 40 μm or smaller and having an average primary particle size of 1 μm or smaller. Containing fine particles having an average primary particle size of 1 μm or smaller leads to an increase in the contact area with the electrolyte solution and more rapid diffusion of lithium ions between the electrode and the electrolyte solution. As a result, the output performance of the battery can be improved.

The positive electrode active material can be produced by any usual method of producing inorganic compounds. In particular, a spherical or ellipsoidal active material can be produced by various methods. For example, a material substance of transition metal is dissolved or crushed and dispersed in a solvent such as water, and the pH of the solution or dispersion is adjusted under stirring to form a spherical precursor. The precursor is recovered and, if necessary, dried. Then, a Li source such as LiOH, Li₂CO₃, or LiNO₃ is added thereto and the mixture is sintered at high temperature, thereby providing an active material.

In order to produce a positive electrode, the aforementioned positive electrode active materials may be used alone or in any combination with two or more having different compositions at any ratio. Preferred examples of the combination in this case include a combination with LiCoO₂ and LiMn₂O₄ in which part of Mn may optionally be replaced by a different transition metal (e.g., LiNi_(0.33)Co_(0.33)Mn_(0.33)O₂), and a combination with LiCoO₂ in which part of Co may optionally be replaced by a different transition metal.

In order to achieve a high battery capacity, the amount of the positive electrode active material is preferably 50 to 99 mass %, more preferably 80 to 99 mass %, of the positive electrode mixture. The amount of the positive electrode active material in the positive electrode active material layer is preferably 80 mass % or more, more preferably 82 mass % or more, particularly preferably 84 mass % or more. The amount thereof is preferably 99 mass % or less, more preferably 98 mass % or less. Too small an amount of the positive electrode active material in the positive electrode active material layer may lead to an insufficient electric capacity. In contrast, too large an amount thereof may lead to an insufficient strength of the resulting positive electrode.

The positive electrode mixture preferably further comprises a binder, a thickening agent, and a conductive material.

The binder may be any material that is safe against a solvent to be used in production of electrodes and the electrolyte solution. Examples thereof include polyvinylidene fluoride, polytetrafluoroethylene, polyethylene, polypropylene, SBR (styrene-butadiene rubber), isoprene rubber, butadiene rubber, ethylene-acrylic acid copolymers, ethylene-methacrylic acid copolymers, polyethylene terephthalate, polymethyl methacrylate, polyimide, aromatic polyamide, cellulose, nitro cellulose, NBR (acrylonitrile-butadiene rubber), fluororubber, ethylene-propylene rubber, styrene-butadiene-styrene block copolymers or hydrogenated products thereof, EPDM (ethylene-propylene-diene terpolymers), styrene-ethylene-butadiene-ethylene copolymers, styrene-isoprene-styrene block copolymers or hydrogenated products thereof, syndictactic-1,2-polybutadiene, polyvinyl acetate, ethylene-vinyl acetate copolymers, propylene-α-olefin copolymers, fluorinated polyvinylidene fluoride, tetrafluoroethylene-ethylene copolymers, and polymer compositions having an ion conductivity of alkali metal ions (especially, lithium ions). These agents may be used alone or in any combination of two or more at any ratio.

The amount of the binder, which is expressed as the proportion of the binder in the positive electrode active material layer, is usually 0.1 mass % or more, preferably 1 mass % or more, more preferably 1.5 mass % or more. The proportion is also usually 80 mass % or less, preferably 60 mass % or less, still more preferably 40 mass % or less, most preferably 10 mass % or less. Too low a proportion of the binder may fail to sufficiently hold the positive electrode active material so that the resulting positive electrode may have an insufficient mechanical strength, resulting in deteriorated battery performance such as cycle characteristics. In contrast, too high a proportion thereof may lead to a reduction in battery capacity and conductivity.

Examples of the thickening agent include carboxymethyl cellulose, methyl cellulose, hydroxymethyl cellulose, ethyl cellulose, polyvinyl alcohol, oxidized starch, monostarch phosphate, casein, and salts thereof. These agents may be used alone or in any combination of two or more at any ratio.

The proportion of the thickening agent relative to the active material is usually 0.1 mass % or more, preferably 0.2 mass % or more, more preferably 0.3 mass % or more. It is usually 5 mass % or less, preferably 3 mass % or less, more preferably 2 mass % or less. If the proportion thereof is below this range, easiness of application may be significantly deteriorated. If the proportion is above this range, the proportion of the active material in the positive electrode active material layer decreases, so that the capacity of the battery may decrease or the resistance between the positive electrode active materials may increase.

The conductive material may be any known conductive material. Specific examples thereof include metal materials such as copper and nickel, and carbon materials such as graphite (e.g., natural graphite, artificial graphite), carbon black (e.g., acetylene black), and amorphous carbon (e.g., needle coke). These materials may be used alone or in any combination of two or more at any ratio. The conductive material is used such that the amount thereof in the positive electrode active material layer is usually 0.01 mass % or more, preferably 0.1 mass % or more, more preferably 1 mass % or more, whereas the amount thereof is usually 50 mass % or less, preferably 30 mass % or less, more preferably 15 mass % or less. If the amount thereof is below this range, the conductivity may be insufficient. In contrast, if the amount thereof is above this range, the battery capacity may decrease.

The solvent for forming slurry may be any solvent that can dissolve or disperse therein the positive electrode active material, the conductive material, and the binder, as well as a thickening agent used if necessary. The solvent may be either of an aqueous solvent or an organic solvent. Examples of the aqueous medium include water and solvent mixtures of an alcohol and water. Examples of the organic medium include aliphatic hydrocarbons such as hexane; aromatic hydrocarbons such as benzene, toluene, xylene, and methyl naphthalene; heterocyclic compounds such as quinoline and pyridine; ketones such as acetone, methyl ethyl ketone, and cyclohexanone; esters such as methyl acetate and methyl acrylate; amines such as diethylene triamine and N,N-dimethylaminopropylamine; ethers such as diethyl ether, propylene oxide, and tetrahydrofuran (THF); amides such as N-methylpyrrolidone (NMP), dimethyl formamide, and dimethyl acetamide; and aprotic polar solvents such as hexamethyl phospharamide and dimethyl sulfoxide.

The current collector for positive electrodes may be formed from any metal material such as aluminum, titanium, tantalum, stainless steel, or nickel, or any alloy thereof; or any carbon material such as carbon cloth or carbon paper. Preferred is any metal material, especially aluminum or alloy thereof.

In the case of a metal material, the current collector may be in the form of metal foil, metal cylinder, metal coil, metal plate, metal film, expanded metal, punched metal, metal foam, or the like. In the case of a carbon material, it may be in the form of carbon plate, carbon film, carbon cylinder, or the like. Preferred among these is a metal film. The film may be in the form of mesh, as appropriate. The film may have any thickness, and the thickness is usually 1 μm or greater, preferably 3 μm or greater, more preferably 5 μm or greater, whereas the thickness is usually 1 mm or smaller, preferably 100 μm or smaller, more preferably 50 μm or smaller. If the film is thinner than this range, it may have an insufficient strength as a current collector. In contrast, if the film is thicker than this range, it may have poor handleability.

In order to reduce the electrical contact resistance between the current collector and the positive electrode active material layer, the current collector also preferably has a conductive auxiliary agent applied on the surface thereof. Examples of the conductive auxiliary agent include carbon and noble metals such as gold, platinum, and silver.

The ratio between the thicknesses of the current collector and the positive electrode active material layer may be any value, and the ratio {(thickness of positive electrode active material layer on one side immediately before injection of electrolyte solution)/(thickness of current collector)} is preferably 20 or lower, more preferably 15 or lower, most preferably 10 or lower. The ratio is also preferably 0.5 or higher, more preferably 0.8 or higher, most preferably 1 or higher. If the ratio exceeds this range, the current collector may generate heat due to Joule heating during high-current-density charge and discharge. If the ratio is below this range, the ratio by volume of the current collector to the positive electrode active material is so high that the capacity of the battery may decrease.

The positive electrode may be produced by a usual method. One example of the production method is a method in which the positive electrode active material is mixed with the aforementioned binder, thickening agent, conductive material, solvent, and other components to form a slurry-like positive electrode mixture, and then this mixture is applied to a current collector, dried, and pressed so as to be densified.

The densification may be achieved using a manual press or a roll press, for example. The density of the positive electrode active material layer is preferably 1.5 g/cm³ or higher, more preferably 2 g/cm³ or higher, still more preferably 2.2 g/cm³ or higher, whereas the density thereof is preferably 5 g/cm³ or lower, more preferably 4.5 g/cm³ or lower, still more preferably 4 g/cm³ or lower. If the density is above this range, the permeability of the electrolyte solution toward the vicinity of the interface between the current collector and the active material may decrease, in particular, the charge and discharge characteristics at high current density may be deteriorated, so that a high output may not be achieved. If the density is below this range, the conductivity between the active materials may decrease and the resistance of the battery may increase, so that a high output may not be achieved.

In order to improve the stability at high output and high temperature, the area of the positive electrode active material layer is preferably large relative to the outer surface area of an external case of the battery in the case of using the electrolyte solution of the present invention. Specifically, the sum of the areas of the positive electrodes is preferably 15 times or more, more preferably 40 times or more, greater than the surface area of the external case of the secondary battery. For closed, square-shaped cases, the outer surface area of an external case of the battery herein refers to the total area calculated from the dimension of length, width, and thickness of the case portion into which a power-generating element is filled except for a protruding portion of a terminal. For closed, cylinder-like cases, the outer surface area of an external case of the battery herein refers to a geometric surface area of an approximated cylinder of the case portion into which a power-generating element is filled except for a protruding portion of a terminal. The sum of the areas of the positive electrodes herein refers to a geometric surface area of a positive electrode mixture layer opposite to a mixture layer including the negative electrode active material. For structures including a current collector foil and positive electrode mixture layers on both sides of the current collector, the sum of the areas of the positive electrodes is the sum of the areas calculated on the respective sides.

The positive electrode plate may have any thickness. In order to achieve a high capacity and a high output, the lower limit of the thickness of the mixture layer on one side of the current collector excluding the thickness of the base metal foil is preferably 10 μm or greater, more preferably 20 μm or greater, whereas the thickness thereof is preferably 500 μm or smaller, more preferably 450 μm or smaller.

To the surface of the positive electrode plate may be attached a substance having a different composition. Examples of the substance attached to the surface include oxides such as aluminum oxide, silicon oxide, titanium oxide, zirconium oxide, magnesium oxide, calcium oxide, boron oxide, antimony oxide, and bismuth oxide; sulfates such as lithium sulfate, sodium sulfate, potassium sulfate, magnesium sulfate, calcium sulfate, and aluminum sulfate; carbonates such as lithium carbonate, calcium carbonate, and magnesium carbonate; and carbon.

<Negative Electrode>

The negative electrode comprises a negative electrode mixture including a negative electrode active material, and a current collector.

Examples of the negative electrode active material include carbonaceous materials that can occlude and release lithium such as pyrolysates of organic matter under various pyrolysis conditions, artificial graphite, and natural graphite; metal oxide materials that can occlude and release lithium such as tin oxide and silicon oxide; lithium metals; various lithium alloys; and lithium-containing metal complex oxide materials. Two or more of these negative electrode active materials may be used in admixture.

The carbonaceous material that can occlude and release lithium is preferably artificial graphite produced by high-temperature treatment of easily graphitizable pitch from various materials, purified natural graphite, or a material obtained by surface-treating such graphite with pitch or other organic matter and then carbonizing the surface-treated graphite. In order to achieve a good balance between the initial irreversible capacity and the high-current-density charge and discharge characteristics, it is more preferably selected from carbonaceous materials obtained by one or more heat treatments at 400° C. to 3200° C. on natural graphite, artificial graphite, artificial carbonaceous substances, or artificial graphite substances; carbonaceous materials which allow the negative electrode active material layer to include at least two or more carbonaceous matters having different crystallinities and/or has an interface between the carbonaceous matters having the different crystallinities; and carbonaceous materials which allow the negative electrode active material layer to have an interface between at least two or more carbonaceous matters having different orientations. These carbonaceous materials may be used alone or in any combination of two or more at any ratio.

Examples of the carbonaceous materials obtained by one or more heat treatments at 400° C. to 3200° C. on artificial carbonaceous substances or artificial graphite substances include coal-based coke, petroleum-based coke, coal-based pitch, petroleum-based pitch, and those prepared by oxidizing these pitches; needle coke, pitch coke, and carbon materials prepared by partially graphitizing these cokes; pyrolysates of organic matter such as furnace black, acetylene black, and pitch-based carbon fibers; carbonizable organic matter and carbides thereof; and solutions prepared by dissolving carbonizable organic matter in a low-molecular-weight organic solvent such as benzene, toluene, xylene, quinoline, or n-hexane, and carbides thereof.

The metal material (excluding lithium-titanium complex oxides) to be used as the negative electrode active material may be any compound that can occlude and release lithium, and examples thereof include simple lithium, simple metals and alloys that constitute lithium alloys, and oxides, carbides, nitrides, silicides, sulfides, and phosphides thereof. The simple metals and alloys constituting lithium alloys are preferably materials containing any of metal and semi-metal elements in the Groups 13 and 14, more preferably simple metal of aluminum, silicon, and tin (hereinafter, referred to as “specific metal elements”), and alloys and compounds containing any of these atoms. These materials may be used alone or in combination of two or more at any ratio.

Examples of the negative electrode active material having at least one atom selected from the specific metal elements include simple metal of any one specific metal element, alloys of two or more specific metal elements, alloys of one or two or more specific metal elements and one or two or more other metal elements, compounds containing one or two or more specific metal elements, and composite compounds such as oxides, carbides, nitrides, silicides, sulfides, and phosphides of the compounds. Use of such a simple metal, alloy, or metal compound as the negative electrode active material can give a high capacity to batteries.

Examples thereof further include compounds in which any of the above composite compounds are complexly bonded with several elements such as simple metals, alloys, and nonmetal elements. Specifically, in the case of silicon or tin, for example, an alloy of this element and a metal that does not serve as a negative electrode can be used. In the case of tin, for example, a composite compound comprising a combination of 5 or 6 elements, including tin, a metal (excluding silicon) that serves as a negative electrode, a metal that does not serve as a negative electrode, and a nonmetal element, can be used.

Specific examples thereof include simple Si, SiB₄, SiB₆, Mg₂Si, Ni₂Si, TiSi₂, MoSi₂, CoSi₂, NiSi₂, CaSi₂, CrSi₂, Cu₆Si, FeSi₂, MnSi₂, NbSi₂, TaSi₂, VSi₂, WSi₂, ZnSi₂, SiC, Si₃N₄, Si₂N₂O, SiOv (0<v≦2), LiSiO, simple tin, SnSiO₃, LiSnO, Mg₂Sn, and SnOw (0<w≦2).

Examples thereof further include composite materials of Si or Sn used as a first constitutional element, and second and third constitutional elements. The second constitutional element is at least one selected from cobalt, iron, magnesium, titanium, vanadium, chromium, manganese, nickel, copper, zinc, gallium, and zirconium, for example. The third constitutional element is at least one selected from boron, carbon, aluminum, and phosphorus, for example.

In order to achieve a high battery capacity and excellent battery characteristics, the metal material is preferably simple silicon or tin (which may contain trace impurities), SiOv (0<v≦2), SnOw (0≦w≦2), a Si—Co—C composite material, a Si—Ni—C composite material, a Sn—Co—C composite material, or a Sn—Ni—C composite material.

The lithium-containing metal complex oxide material to be used as the negative electrode active material may be any material that can occlude and release lithium. In order to achieve good high-current-density charge and discharge characteristics, materials containing titanium and lithium are preferred, lithium-containing metal complex oxide materials containing titanium are more preferred, and complex oxides of lithium and titanium (hereinafter, abbreviated as “lithium titanium complex oxides”) are still more preferred. In other words, use of a spinel-structured lithium titanium complex oxide contained in the negative electrode active material for electrolyte batteries is particularly preferred because such a compound markedly reduces the output resistance.

Preferred examples of the lithium titanium complex oxides include compounds represented by the formula (O):

Li_(x)Ti_(y)M₂O₄  (O)

wherein M represents at least one element selected from the group consisting of Na, K, Co, Al, Fe, Ti, Mg, Cr, Ga, Cu, Zn, and Nb.

Particularly preferred compositions represented by the formula (O) are those satisfying one of the following:

(i) 1.2≦x≦1.4, 1.5≦y≦1.7, z=0 (ii) 0.9≦x≦1.1, 1.9≦y≦2.1, z=0 (iii) 0.7≦x≦0.9, 2.1≦y≦2.3, z=0 because the compound structure satisfying any of these compositions give good balance of the battery performance.

Particularly preferred representative composition of the compound is Li_(4/3)Ti_(5/3)O₄ corresponding to the composition (i), Li₁Ti₂O₄ corresponding to the composition (ii), and Li_(4/5)Ti_(11/5)O₄ corresponding to the composition (iii). Preferred examples of the structure satisfying Z≠0 include Li_(4/3)Ti_(4/3)Al_(1/3)O₄.

The negative electrode mixture preferably further contains a binder, a thickening agent, and a conductive material.

Examples of the binder include the same binders as those mentioned for the positive electrode. The proportion of the binder relative to the negative electrode active material is preferably 0.1 mass % or more, more preferably 0.5 mass % or more, particularly preferably 0.6 mass % or more. The proportion is also preferably 20 mass % or less, more preferably 15 mass % or less, still more preferably 10 mass % or less, particularly preferably 8 mass % or less. If the proportion of the binder relative to the negative electrode active material exceeds the above range, a large amount of the binder may fail to contribute to the battery capacity, so that the battery capacity may decrease. If the proportion is lower than the above range, the negative electrode may have a lowered strength.

In particular, in the case of using a rubbery polymer typified by SBR as a main component, the proportion of the binder relative to the negative electrode active material is usually 0.1 mass % or more, preferably 0.5 mass % or more, more preferably 0.6 mass % or more, whereas the proportion thereof is usually 5 mass % or less, preferably 3 mass % or less, more preferably 2 mass % or less. In the case of using a fluoropolymer typified by polyvinylidene fluoride as a main component, the proportion of the binder relative to the negative electrode active material is usually 1 mass % or more, preferably 2 mass % or more, more preferably 3 mass % or more, whereas the proportion thereof is usually 15 mass % or less, preferably 10 mass % or less, more preferably 8 mass % or less.

Examples of the thickening agent include the same thickening agents as those mentioned for the positive electrode. The proportion of the thickening agent relative to the negative electrode active material is usually 0.1 mass % or more, preferably 0.5 mass % or more, still more preferably 0.6 mass % or more, whereas the proportion thereof is usually 5 mass % or less, preferably 3 mass % or less, still more preferably 2 mass % or less. If the proportion of the thickening agent relative to the negative electrode active material is below the range, easiness of application may be significantly deteriorated. If the proportion thereof is above the range, the proportion of the negative electrode active material in the negative electrode active material layer decreases, so that the capacity of the battery may decrease or the resistance between the negative electrode active materials may increase.

Examples of the conductive material of the negative electrode include metal materials such as copper and nickel; and carbon materials such as graphite and carbon black.

The solvent for forming slurry may be any solvent that can dissolve or disperse the negative electrode active material and the binder, and a thickening agent and a conductive material that are used as necessary. The slurry-forming solvent may be an aqueous solvent or an organic solvent.

Examples of the aqueous solvent include water and alcohols. Examples of the organic solvent include N-methylpyrrolidone (NMP), dimethyl formamide, dimethyl acetamide, methyl ethyl ketone, cyclohexanone, methyl acetate, methyl acrylate, diethyl triamine, N,N-dimethyl aminopropyl amine, tetrahydrofuran (THF), toluene, acetone, diethyl ether, dimethyl acetamide, hexamethyl phospharamide, dimethyl sulfoxide, benzene, xylene, quinoline, pyridine, methyl naphthalene, and hexane.

Examples of the material of the current collector for negative electrodes include copper, nickel, and stainless steel. For easy processing of the material into a film and low cost, copper is preferred.

The current collector usually has a thickness of 1 μm or larger, preferably 5 μm or larger. The thickness is also usually 100 m or smaller, preferably 50 μm or smaller. Too thick a negative electrode current collector may cause an excessive reduction in capacity of the whole battery, whereas too thin a current collector may be difficult to handle.

The negative electrode may be produced by a usual method. One example of the production method is a method in which the negative electrode material is mixed with the aforementioned binder, thickening agent, conductive material, solvent, and other components to form a slurry-like mixture, and then this mixture is applied to a current collector, dried, and pressed so as to be densified. In the case of using an alloyed material, a thin film layer containing the above negative electrode active material (negative electrode active material layer) can be produced by vapor deposition, sputtering, plating, or the like technique.

The electrode formed from the negative electrode active material may have any structure. The density of the negative electrode active material existing on the current collector is preferably 1 g·cm⁻³ or higher, more preferably 1.2 g·cm⁻³ or higher, particularly preferably 1.3 g·cm⁻³ or higher, whereas the density thereof is preferably 2.2 g·cm⁻³ or lower, more preferably 2.1 g·cm⁻³ or lower, still more preferably 2.0 g·cm⁻³ or lower, particularly preferably 1.9 g·cm⁻³ or lower. If the density of the negative electrode active material existing on the current collector exceeds the above range, the particles of the negative electrode active material may be broken, the initial irreversible capacity may increase, and the permeability of the electrolyte solution toward the vicinity of the interface between the current collector and the negative electrode active material may be deteriorated, so that the high-current-density charge and discharge characteristics may be deteriorated. If the density thereof is below the above range, the conductivity between the negative electrode active materials may be deteriorated, the resistance of the battery may increase, and the capacity per unit volume may decrease.

The thickness of the negative electrode plate is a design matter in accordance with the positive electrode plate to be used, and may be any value. The thickness of the mixture layer excluding the thickness of the base metal foil is usually 15 μm or greater, preferably 20 μm or greater, more preferably 30 μm or greater, whereas the thickness thereof is usually 300 μm or smaller, preferably 280 μm or smaller, more preferably 250 μm or smaller.

To the surface of the negative electrode plate may be attached a substance having a different composition. Examples of the substance attached to the surface include oxides such as aluminum oxide, silicon oxide, titanium oxide, zirconium oxide, magnesium oxide, calcium oxide, boron oxide, antimony oxide, and bismuth oxide; sulfates such as lithium sulfate, sodium sulfate, potassium sulfate, magnesium sulfate, calcium sulfate, and aluminum sulfate; and carbonates such as lithium carbonate, calcium carbonate, and magnesium carbonate.

<Separator>

The lithium ion secondary battery preferably further comprises a separator.

The separator may be formed from any known material and may have any known shape as long as the resulting separator is stable to the electrolyte solution and is excellent in a liquid-retaining ability. The separator is preferably in the form of a porous sheet or a nonwoven fabric which is formed from a material stable to the electrolyte solution of the present invention, such as resin, glass fiber, or inorganic matter, and which is excellent in a liquid-retaining ability.

Examples of the material of a resin or glass-fiber separator include polyolefins such as polyethylene and polypropylene, aromatic polyamide, polytetrafluoroethylene, polyether sulfone, and glass filters. These materials may be used alone or in any combination of two or more at any ratio, for example, in the form of a polypropylene/polyethylene bilayer film or a polypropylene/polyethylene/polypropylene trilayer film. In order to achieve good permeability of the electrolyte solution and a good shut-down effect, the separator is particularly preferably a porous sheet or a nonwoven fabric formed from polyolefin such as polyethylene or polypropylene.

The separator may have any thickness, and the thickness is usually 1 μm or larger, preferably 5 μm or larger, more preferably 8 μm or larger, whereas the thickness is usually 50 μm or smaller, preferably 40 μm or smaller, more preferably 30 μm or smaller. If the separator is thinner than the above range, the insulation and mechanical strength may be poor. If the separator is thicker than the above range, not only the battery performance, such as rate characteristics, may be poor but also the energy density of the whole electrolyte battery may be low.

If the separator is a porous one such as a porous sheet or a nonwoven fabric, the separator may have any porosity. The porosity is usually 20% or higher, preferably 35% or higher, more preferably 45% or higher, whereas the porosity is usually 90% or lower, preferably 85% or lower, more preferably 75% or lower. If the porosity is lower than the range, the film resistance tends to be high and the rate characteristics tend to be poor. If the porosity is higher than the range, the mechanical strength of the separator tends to be low and the insulation tends to be poor.

The separator may also have any average pore size. The average pore size is usually 0.5 μm or smaller, preferably 0.2 μm or smaller, whereas the average pore size is usually 0.05 μm or larger. If the average pore size exceeds the range, short circuits may easily occur. If the average pore size is lower than the range, the film resistance may be high and the rate characteristics may be poor.

Examples of the inorganic material include oxides such as alumina and silicon dioxide, nitrides such as aluminum nitride and silicon nitride, and sulfates such as barium sulfate and calcium sulfate. The inorganic material is in the form of particles or fibers.

The separator is in the form of a thin film such as a nonwoven fabric, a woven fabric, or a microporous film. The thin film favorably has a pore size of 0.01 to 1 μm and a thickness of 5 to 50 μm. In addition to the form of the above separate thin film, the separator may have a structure in which a composite porous layer containing particles of the above inorganic material is formed on the surface of one or both of the positive and negative electrodes using a resin binder. For example, alumina particles having a 90% particle size of smaller than 1 μm is applied to the respective surfaces of the positive electrode with fluororesin used as a binder to form a porous layer.

<Battery Design>

The electrode group may be either a laminated structure comprising the above positive and negative electrode plates with the above separator in between, or a wound structure comprising the above positive and negative electrode plates in spiral with the above separator in between. The proportion of the volume of the electrode group in the battery internal volume (hereinafter, referred to as an electrode group proportion) is usually 40% or higher, preferably 50% or higher, whereas the proportion thereof is usually 90% or lower, preferably 80% or lower.

If the electrode group proportion is lower than the above range, the battery capacity may be low. If the electrode group proportion exceeds the above range, the battery may have small space for voids. Thus, when the battery temperature rises to high temperature, the components may swell or the liquid fraction of the electrolyte solution shows a high vapor pressure, so that the internal pressure rises. As a result, the battery characteristics such as charge and discharge repeatability and the high-temperature storageability may be deteriorated, and a gas-releasing valve for releasing the internal pressure toward the outside may work.

The current collecting structure may be any structure. In order to more effectively improve the high-current-density charge and discharge characteristics by the electrolyte solution of the present invention, the current collecting structure is preferably a structure which reduces the resistances at wiring portions and jointing portions. When the internal resistance is reduced in such a manner, the effects of using the electrolyte solution of the present invention can particularly favorably be achieved.

In an electrode group having the layered structure, the metal core portions of the respective electrode layers are preferably bundled and welded to a terminal. If an electrode has a large area, the internal resistance is high. Thus, multiple terminals may preferably be formed in the electrode to reduce the resistance. In an electrode group having the wound structure, multiple lead structures may be disposed on each of the positive electrode and the negative electrode and bundled to a terminal. Thereby, the internal resistance can be reduced.

The external case may be made of any material that is stable to an electrolyte solution to be used. Specific examples thereof include metals such as nickel-plated steel plates, stainless steel, aluminum and aluminum alloys, and magnesium alloys, and layered film (laminate film) of resin and aluminum foil. In order to reduce the weight, a metal such as aluminum or an aluminum alloy or a laminate film is favorably used.

External cases made of metal may have a sealed up structure formed by welding the metal by laser welding, resistance welding, or ultrasonic welding or a caulking structure using the metal via a resin gasket. External cases made of a laminate film may have a sealed up structure formed by hot melting the resin layers. In order to improve the sealability, a resin which is different from the resin of the laminate film may be disposed between the resin layers. Especially, in the case of forming a sealed up structure by hot melting the resin layers via current collecting terminals, metal and resin are to be bonded. Thus, the resin to be disposed between the resin layers is favorably a resin having a polar group or a modified resin having a polar group introduced thereinto.

The lithium ion secondary battery may have any shape, and examples thereof include cylindrical batteries, square batteries, laminated batteries, coin batteries, and large-size batteries. The shapes and the configurations of the positive electrode, the negative electrode, and the separator may be changed in accordance with the shape of the battery.

A module comprising the electrochemical device or secondary battery that comprises the electrolyte solution of the present invention is also one aspect of the present invention.

Examples of the electrochemical device using the electrolyte solution of the present invention include an electric double-layer capacitor.

In the electric double-layer capacitor, at least one of the positive electrode and the negative electrode is a polarizable electrode. Examples of the polarizable electrode and a non-polarizable electrode include the following electrodes specifically disclosed in JP H09-7896 A.

The polarizable electrode mainly containing activated carbon is preferably one containing inactivated carbon having a large specific surface area and a conductive material, such as carbon black, providing electronic conductivity. The polarizable electrode can be formed by any of various methods. For example, a polarizable electrode comprising activated carbon and carbon black can be produced by mixing activated carbon powder, carbon black, and phenolic resin, press-molding the mixture, and then sintering and activating the mixture in an inert gas atmosphere and water vapor atmosphere. Preferably, this polarizable electrode is bonded to a current collector using a conductive adhesive, for example.

Alternatively, a polarizable electrode can also be formed by kneading activated carbon powder, carbon black, and a binder in the presence of alcohol and forming the mixture into a sheet shape, and then drying the sheet. This binder may be polytetrafluoroethylene, for example. Alternatively, a polarizable electrode integrated with a current collector can be produced by mixing activated carbon powder, carbon black, a binder, and a solvent to form slurry, applying this slurry to metal foil of a current collector, and then drying the slurry.

The electric double-layer capacitor may have polarizable electrodes mainly containing activated carbon on the respective sides. Still, the electric double-layer capacitor may have a non-polarizable electrode on one side, for example, a positive electrode mainly comprising an electrode active material such as a metal oxide and a negative electrode comprising a polarizable electrode that mainly contains activated carbon may be combined; or a negative electrode mainly comprising a carbon material that can reversibly occlude and release lithium ions or a negative electrode of lithium metal or lithium alloy and a polarizable positive electrode mainly comprising activated carbon may be combined.

In place of or in combination with activated carbon, any carbonaceous material such as carbon black, graphite, expanded graphite, porous carbon, carbon nanotube, carbon nanohorn, and Kethenblack may be used.

The non-polarizable electrode is preferably an electrode mainly containing a carbon material that can reversibly occlude and release lithium ions, and this carbon material is made to occlude lithium ions in advance. In this case, the electrolyte used is a lithium salt. The electric double-layer capacitor having such a configuration achieves a much higher withstand voltage of exceeding 4 V.

The solvent used in preparation of slurry in the production of electrodes is preferably one that dissolves a binder. In accordance with the type of a binder, N-methylpyrrolidone, dimethyl formamide, toluene, xylene, isophorone, methyl ethyl ketone, ethyl acetate, methyl acetate, dimethyl phthalate, ethanol, methanol, butanol, or water is appropriately selected.

Examples of the activated carbon used for the polarizable electrode include phenol resin-type activated carbon, coconut shell-type activated carbon, and petroleum coke-type activated carbon. In order to achieve a large capacity, petroleum coke-type activated carbon or phenol resin-type activated carbon is preferably used. Examples of methods of activating the activated carbon include steam activation and molten KOH activation. In order to achieve a larger capacity, activated carbon prepared by molten KOH activation is preferably used.

Preferred examples of the conductive material used for the polarizable electrode include carbon black, Ketjenblack, acetylene black, natural graphite, artificial graphite, metal fiber, conductive titanium oxide, and ruthenium oxide. In order to achieve good conductivity (i.e., low internal resistance), and because too large an amount thereof may lead to a decreased capacity of the product, the amount of the conductive material such as carbon black used for the polarizable electrode is preferably 1 to 50 mass % in the sum of the amounts of the activated carbon and the conductive material.

In order to provide an electric double-layer capacitor having a large capacity and a low internal resistance, the activated carbon used for the polarizable electrode preferably has an average particle size of 20 μm or smaller and a specific surface area of 1500 to 3000 m²/g. Preferred examples of the carbon material for providing an electrode mainly containing a carbon material that can reversibly occlude and release lithium ions include natural graphite, artificial graphite, graphitized mesocarbon microsphere, graphitized whisker, vapor-grown carbon fiber, sintered furfuryl alcohol resin, and sintered novolak resin.

The current collector may be any chemically and electrochemically corrosion-resistant one. Preferred examples of the current collector used for the polarizable electrode mainly containing activated carbon include stainless steel, aluminum, titanium, and tantalum. Particularly preferred materials in terms of the characteristics and cost of the resulting electric double-layer capacitor are stainless steel and aluminum. Preferred examples of the current collector used for the electrode mainly containing a carbon material that can reversibly occlude and release lithium ions include stainless steel, copper, and nickel.

The carbon material that can reversibly occlude and release lithium ions can be allowed to occlude lithium ions in advance by (1) a method of mixing powdery lithium to a carbon material that can reversibly occlude and release lithium ions, (2) a method of placing lithium foil on an electrode comprising a carbon material that can reversibly occlude and release lithium ions and a binder so that the lithium foil is electrically in contact with the electrode, immersing this electrode in an electrolyte solution containing a lithium salt dissolved therein so that the lithium is ionized, and allowing the carbon material to take in the resulting lithium ions, or (3) a method of placing an electrode comprising a carbon material that can reversibly occlude and release lithium ions and a binder at a minus side and placing a lithium metal at a plus side, immersing the electrodes in a non-aqueous electrolyte solution containing a lithium salt as an electrolyte, and supplying a current so that the carbon material is allowed to electrochemically take in the ionized lithium.

Examples of known electric double-layer capacitors include wound electric double-layer capacitors, laminated electric double-layer capacitors, and coin-type electric double-layer capacitors. The electric double-layer capacitor of the present invention may also be any of these types.

For example, a wound electric double-layer capacitor is assembled by winding a positive electrode and a negative electrode each of which comprises a laminate (electrode) of a current collector and an electrode layer, and a separator in between to provide a wound element, putting this wound element in a case made of, for example, aluminum, filling the case with an electrolyte solution, preferably a non-aqueous electrolyte solution, and sealing the case with a rubber sealant.

In the present invention, a separator formed from a conventionally known material and having a conventionally known structure can also be used. Examples thereof include polyethylene porous membranes, and nonwoven fabric of polypropylene fiber, glass fiber, or cellulose fiber.

In accordance with any known method, the capacitor may be formed into a laminated electric double-layer capacitor in which a sheet-like positive electrode and negative electrode are stacked with an electrolyte solution and a separator in between or a coin-type electric double-layer capacitor in which a positive electrode and a negative electrode are fixed by a gasket with an electrolyte solution and a separator in between.

As mentioned above, use of the electrolyte solution of the present invention can suitably provide secondary batteries excellent in high-temperature storage characteristics, modules using the secondary batteries, and electric double-layer capacitors.

EXAMPLES

In the following, the present invention will be more specifically described referring to, but not limited to, examples.

Experiment 1 Use of Fluorine-Containing Acyclic Carbonate as Additive

The electrolyte solutions of Examples 1 to 16 and Comparative Examples 1 to 5 were prepared through the following process, and the amount of gas of each electrolyte solution was measured. A lithium ion secondary battery was produced using each electrolyte solution and the storage capacity retention ratio thereof was evaluated.

(Preparation of Electrolyte Solution)

Under a dry argon atmosphere, ethyl methyl carbonate (EMC) and ethylene carbonate (EC) were mixed at a volume ratio of 70:30. Dry LiPF₆ was dissolved in the resulting solution at 1 mol/L, and thereby a base electrolyte solution (4.35 V) was obtained. To the base electrolyte solution were added a fluorine-containing acyclic carbonate and a fluorine-containing succinic anhydride in the mix proportions (mass %) shown in Table 1. Thereby, the electrolyte solutions of Examples 1 to 16 and Comparative Examples 1 to 5 were prepared. Table 1 shows the results.

The components shown in Table 1 are as follows.

Component (A): CH₃OCOOCH₂CF₃ Component (B): CF₃CH₂OCOOCH₂CF₃

(Production of Negative Electrode)

Powder of artificial graphite as a negative electrode active material, an aqueous dispersion of carboxymethyl cellulose sodium (concentration of carboxymethyl cellulose sodium: 1 mass %) as a thickening agent, and an aqueous dispersion of styrene-butadiene rubber (concentration of styrene-butadiene rubber: 50 mass %) as a binder were mixed at a ratio (by mass %) of 97.6/1.2/1.2 in a water solvent to prepare a negative electrode mixture slurry. The slurry was uniformly applied to 20-μm-thick copper foil, followed by drying, and then the workpiece was formed into a negative electrode by compression molding with a press.

(Production of Positive Electrode)

A positive electrode material in which LiN_(1/3)Mn_(1/3)Co_(1/3)O₂ as a positive electrode active material, acetylene black as a conductive material, and polyvinylidene fluoride (PVdF) as a binder were mixed at a ratio (by mass %) of 92/3/5 was dispersed in N-methyl-2-pyrrolidone to prepare a positive electrode mixture slurry. The positive electrode mixture slurry was uniformly applied to a 21-μm-thick current collector made of aluminum foil, followed by drying, to form a positive electrode mixture layer. The workpiece was then formed into a positive electrode by compression molding with a press.

(Production of Lithium Ion Secondary Battery)

A battery element was produced by stacking the above prepared negative electrode, a polyethylene separator, and the above prepared positive electrode in the given order.

A bag made of a laminate film in which an aluminum sheet (thickness: 40 μm) was coated with a resin layer on each side was prepared. The above battery element was placed in the bag in such a manner that the terminals of the positive electrode and negative electrode stuck out of the bag. Each of the electrolyte solutions of Examples 1 to 16 and Comparative Examples 1 to 5 was poured into the bag and the bag was vacuum sealed. Thereby, a lithium ion secondary battery in a sheet form was produced.

<Evaluation Test for High-Temperature Storage Characteristics>

The above produced secondary battery sandwiched and pressurized between plates was subjected to constant current-constant voltage charge (hereinafter, referred to as CC/CV charge) (0.1 C cut off) to 4.35 V at a current corresponding to 0.2 C at 25° C. Then, the battery was discharged to 3 V at a constant current of 0.2 C. This process was counted as one cycle. The initial discharge capacity was determined from the discharge capacity of the third cycle. Here, 1 C means a current value required for discharging the reference capacity of a battery in an hour. For example, 0.2 C indicates a ⅕ current value thereof. The battery was subjected to CC/CV charge (0.1 C cut off) again to 4.35 V, and then stored at a temperature as high as 60° C. for three days. The battery was sufficiently cooled, and then the volume of the battery was measured by the Archimedes' method. From the volume change between before and after the storage, the amount of gas generated was determined. Next, the battery was discharged to 3 V at 0.2 C and 25° C., and the remaining capacity after high-temperature storage was measured. Thereby, the ratio of the remaining capacity to the initial discharge capacity was determined, which was regarded as a storage capacity retention ratio (%).

(Remaining capacity)÷(initial discharge capacity)×100=storage capacity retention ratio (%)

TABLE 1 Compound mixed with base electrolyte solution Storage Fluorine-containing Fluorine-containing capacity acyclic carbonate succinic anhydride Amount retention Mix proportion Mix proportion of gas ratio Composition (mass %) Composition (mass %) (mL) (%) Example 1 Component (A) 0.5 Component (I) 0.001 0.33 74 Example 2 Component (A) 0.5 Component (I) 0.01 0.30 76 Example 3 Component (A) 0.5 Component (I) 0.1 0.28 80 Example 4 Component (A) 0.5 Component (I) 0.5 0.25 83 Example 5 Component (A) 0.5 Component (I) 6 0.25 81 Example 6 Component (A) 0.5 Component (I) 8 0.26 77 Example 7 Component (A) 0.5 Component (I) 20 0.30 75 Example 8 Component (A) 0.001 Component (I) 0.5 0.34 74 Example 9 Component (A) 0.01 Component (I) 0.5 0.33 75 Example 10 Component (A) 0.1 Component (I) 0.5 0.33 79 Example 11 Component (A) 6 Component (I) 0.5 0.30 79 Example 12 Component (A) 8 Component (I) 0.5 0.31 77 Example 13 Component (A) 17 Component (I) 0.5 0.32 73 Example 14 Component (A) 0.5 Component (II) 0.5 0.35 81 Example 15 Component (A) 0.5 Component (III) 0.5 0.35 82 Example 16 Component (B) 0.5 Component (I) 0.5 0.33 82 Comparative — — — — 0.46 60 Example 1 Comparative — — Component (I) 0.5 0.42 67 Example 2 Comparative Component (A) 0.5 — — 0.40 62 Example 3 Comparative Component (A) 0.5 Component (I) 30 0.38 68 Example 4 Comparative Component (A) 0.5 Component (IV) 0.5 0.38 69 Example 5

The table shows that the amounts of gas generated from the electrolyte solutions of Examples 1 to 16 were less than those of Comparative Examples 1 to 5. The table also shows that lithium ion secondary batteries using the electrolyte solutions of Examples 1 to 16 had better storage capacity retention ratios than lithium ion secondary batteries using the electrolyte solutions of Comparative Examples 1 to 5.

Experiment 2 Use of Fluorine-Containing Acyclic Carbonate as Solvent

The electrolyte solutions of Examples 17 to 32 and Comparative Examples 6 to 11 were prepared through the following process, and the amount of gas of each electrolyte solution was measured. A lithium ion secondary battery was produced using each electrolyte solution and the storage capacity retention ratio thereof was evaluated.

(Preparation of Electrolyte Solution)

Under a dry argon atmosphere, the above component (A) or (B) and fluoroethylene carbonate (FEC) were mixed at a volume ratio of X:(100−X). Dry LiPF₆ was dissolved in the resulting solution at 1 mol/L, and thereby a base electrolyte solution (4.85 V) was obtained. To the base electrolyte solution was added a fluorine-containing succinic anhydride in the mix proportions (mass %) shown in Table 2. Thereby, the electrolyte solutions of Examples 17 to 32 and Comparative Examples 6 to 11 were prepared. Table 2 shows the results.

The components shown in Table 2 are the same as those in Table 1.

(Production of Negative Electrode)

Powder of artificial graphite as a negative electrode active material, an aqueous dispersion of carboxymethyl cellulose sodium (concentration of carboxymethyl cellulose sodium: 1 mass %) as a thickening agent, and an aqueous dispersion of styrene-butadiene rubber (concentration of styrene-butadiene rubber: 50 mass %) as a binder were mixed at a ratio (by mass %) of 97.6/1.2/1.2 in a water solvent to prepare a negative electrode mixture slurry. The slurry was uniformly applied to 20-μm-thick copper foil, followed by drying, and then the workpiece was formed into a negative electrode by compression molding with a press.

(Production of Positive Electrode)

A positive electrode material in which LiNi_(0.5)Mn_(0.5)O₄ as a positive electrode active material, acetylene black as a conductive material, and polyvinylidene fluoride (PVdF) as a binder were mixed at a ratio (by mass %) of 92/3/5 was dispersed in N-methyl-2-pyrrolidone to prepare a positive electrode mixture slurry. The positive electrode mixture slurry was uniformly applied to a 21-μm-thick current collector made of aluminum foil, followed by drying, to form a positive electrode mixture layer. The workpiece was then formed into a positive electrode by compression molding with a press.

(Production of Lithium Ion Secondary Battery)

A battery element was produced by stacking the above prepared negative electrode, a polyethylene separator, and the above prepared positive electrode in the given order.

A bag made of a laminate film in which an aluminum sheet (thickness: 40 μm) was coated with a resin layer on each side was prepared. The above battery element was placed in the bag in such a manner that the terminals of the positive electrode and negative electrode stuck out of the bag. Each of the electrolyte solutions of Examples 17 to 32 and Comparative Examples 6 to 11 was poured into the bag and the bag was vacuum sealed. Thereby, a lithium ion secondary battery in a sheet form was produced.

<Evaluation Test for High-Temperature Storage Characteristics>

The above produced secondary battery sandwiched and pressurized between plates was subjected to constant current-constant voltage charge (hereinafter, referred to as CC/CV charge) (0.1 C cut off) to 4.85 V at a current corresponding to 0.2 C at 25° C. Then, the battery was discharged to 3 V at a constant current of 0.2 C. This process was counted as one cycle. The initial discharge capacity was determined from the discharge capacity of the third cycle. Here, 1 C means a current value required for discharging the reference capacity of a battery in an hour. For example, 0.2 C indicates a ⅕ current value thereof. The battery was subjected to CC/CV charge (0.1 C cut off) again to 4.85 V, and then stored at a temperature as high as 60° C. for three days. The battery was sufficiently cooled, and then the volume of the battery was measured by the Archimedes' method. From the volume change between before and after the storage, the amount of gas generated was determined. Next, the battery was discharged to 3 V at 0.2 C and 25° C., and the remaining capacity after high-temperature storage was measured. Thereby, the ratio of the remaining capacity to the initial discharge capacity was determined, which was regarded as a storage capacity retention ratio (%).

(Remaining capacity)+(initial discharge capacity)×100=storage capacity retention ratio (%)

TABLE 2 Compound mixed with base electrolyte solution Storage Fluorine-containing Fluorine-containing capacity acyclic carbonate succinic anhydride Amount retention Mix proportion X Mix proportion of gas ratio Composition (vol %) Composition (mass %) (mL) (%) Example 17 Component (A) 70 Component (I) 0.001 4.2 74 Example 18 Component (A) 70 Component (I) 0.01 4.1 76 Example 19 Component (A) 70 Component (I) 0.1 4.0 80 Example 20 Component (A) 70 Component (I) 0.5 3.8 80 Example 21 Component (A) 70 Component (I) 6 3.7 78 Example 22 Component (A) 70 Component (I) 8 3.6 76 Example 23 Component (A) 70 Component (I) 20 3.6 75 Example 24 Component (A) 20 Component (I) 0.5 4.3 70 Example 25 Component (A) 25 Component (I) 0.5 4.2 72 Example 26 Component (A) 30 Component (I) 0.5 4.1 76 Example 27 Component (A) 75 Component (I) 0.5 3.5 77 Example 28 Component (A) 80 Component (I) 0.5 3.4 76 Example 29 Component (A) 85 Component (I) 0.5 3.4 73 Example 30 Component (A) 70 Component (II) 0.5 3.9 79 Example 31 Component (A) 70 Component (III) 0.5 3.9 79 Example 32 Component (B) 70 Component (I) 0.5 3.9 78 Comparative — — — — 6.2 59 Example 6 Comparative — — Component (I) 0.5 4.8 60 Example 7 Comparative Component (A) 92 Component (I) 0.5 4.3 55 Example 6 Comparative Component (A) 70 — — 4.4 62 Example 9 Comparative Component (A) 70 Component (I) 30 3.9 68 Example 10 Comparative Component (A) 70 Component (IV) 0.5 5.8 40 Example 11

The table shows that the amounts of gas generated from the electrolyte solutions of Examples 17 to 32 were less than those of Comparative Examples 6 to 11. The table also shows that lithium ion secondary batteries using the electrolyte solutions of Examples 17 to 32 had better storage capacity retention ratios than lithium ion secondary batteries using the electrolyte solutions of Comparative Examples 6 to 11.

INDUSTRIAL APPLICABILITY

The electrolyte solution of the present invention can be suitably used as an electrolyte solution for electrochemical devices such as lithium ion secondary batteries. 

1. An electrolyte solution comprising a solvent and an electrolyte salt, the solvent comprising a fluorine-containing acyclic carbonate represented by the formula (1) and a fluorine-containing succinic anhydride represented by the formula (2), the electrolyte solution containing not less than 0.001 mass % but less than 90 vol % of the fluorine-containing acyclic carbonate, the electrolyte solution containing 0.001 to 20 mass % of the fluorine-containing succinic anhydride, the formula (1) being: Rf¹OCOORf²  (1) wherein Rf¹ and Rf² are the same as or different from each other and individually represent a C1-C4 alkyl group or a C1-C4 fluorine-containing alkyl group, but at least one of Rf¹ and Rf² is C1-C4 fluorine-containing alkyl group, the formula (2) being:

wherein Rf³ to Rf⁶ are the same as or different from one another and individually represent a hydrogen atom, a fluorine atom, or a C1-C10 fluorine-containing alkyl group, but not all of Rf³ to Rf⁶ are hydrogen atoms.
 2. The electrolyte solution according to claim 1, wherein the fluorine-containing acyclic carbonate is at least one selected from the group consisting of CF₃CH₂OCOOCH₂CF₃, CF₃CH₂OCOOCH₃, and CF₃CF₂CH₂OCOOCH₂CF₂CF₃.
 3. The electrolyte solution according to claim 1, wherein the fluorine-containing succinic anhydride is at least one selected from the group consisting of compounds represented by the following formulas (3) to (5):


4. The electrolyte solution according to claim 1, wherein the solvent further comprises at least one selected from the group consisting of a non-fluorinated saturated cyclic carbonate, a fluorinated saturated cyclic carbonate, and a non-fluorinated acyclic carbonate.
 5. The electrolyte solution according to claim 1, wherein the electrolyte salt is at least one lithium salt selected from the group consisting of LiPF₆, LiBF₄, LiSO₃CF₃, LiN(SO₂CF₃)₂, LiN(SO₂C₂F₅)₂, lithium difluoro(oxalato)borate, lithium bis(oxalato)borate, and a salt represented by LiPF_(a)(C_(n)F_(2n+1))_(6-a) wherein a represents an integer of 0 to 5 and n represents an integer of 1 to
 6. 6. An electrochemical device comprising the electrolyte solution according to claim
 1. 7. A secondary battery comprising the electrolyte solution according to claim
 1. 8. A module comprising the electrochemical device according to claim
 6. 